Cloning a tabletop RPG ain't fucking hard, folks. Especially if you're not looking to publish it, but just use it at home. As proof of concept I'll do a bare bones clone of a Palladium Books game, just enough to get started playing at home.
You're using the d20 System as your engine. Don't even bother with anything else. You can count the tabletop RPGs that aren't really D&D in other-genre drag or have some different rules bolted on with one hand, and the rest are a bunch of wankerriffic unplayable crap you're better off burning for fuel or wiping your ass with, so don't fight it- make a virtue of necessity and just fucking use d20.
What you are NOT doing is using it blindly. You're going to strip this motherfucker down to its fundamental elements. For our purposes, that means the following:
- The Six Standard Stats: STR, INT, WIS, DEX, CON, CHA. (Yes, we're junking Physical Beauty; no one cares and its useless, where what matters is Reaction Adjustment and that's Charisma. Speed is separate entirely.)
- The Three Standard Saves: Fortitude, Reflex, Will. Don't need anything else.
- The Core Mechanic: d20+Bonus vs. Target Number. Always Roll High.
See that? Lots of stuff folks expect as normal got junked: classes, levels, skills, Hit Points, etc. all tossed. Why? Don't need them!
Classes got junked because this is, in practice, a single-class game: Everyone's a Mech Pilot. Levels got junked because they don't do what you need from them for this game. Skills are irrelevant as-written; what matters is what you're trained to do, and that requires time and effort. The same is used for damage or injury and recovery; you need time, materials, and a secure location. That also means that XP gets junked; you want to improve? Then you have your man mark out the time necessary to put in the work, and then do it- just like real life. Furthermore, certain basics are just assumed; these assumptions, in a published work, would be specified and spelled out so there's no time wasted arguing about it.
Mechs work as wholesale stat block replacements, and there is a scaling system in place for Man/Mech interactions.
Damage is a flat out Fortitude Saving Throw, meaning each attach is Save or Suck if not Save or Die. Attacks are vs. the target's Reflex Save, by default an opposed roll; vs. mooks that should just be Save+10. Armor flat-out negates attacks, provides a bonus to the save, or both. Scaling differences have similar effects, often in both directions. Specific damage effects (disarms, etc.) are adjudicated at the table by the GM as he sees fit.
Character Generation: Roll 2d4+8 in the above order. The minimum score ensures baseline competency, while selecting for above-average scores as the norm (vs. the usual 3d6 distribution), which is necessary for mech pilots. All character are the same species, and assumed to be male aged 16-21 (1d6+15). Your pilot is Trained as a Soldier, a Fighter Pilot, an Astronaunt, and a Mech Pilot.
- Fundamentals: Competent with English, written and spoken. Competent with basic arithmetic, physics, geometry, calculus, and trigonometry.
- Soldier: Basic military training assumed. Is physically fit, has basic unarmed melee combat training supplemented with knife, club (baton) and spear (bayonet) training. Competent with pistols, shotguns, and rifles. Knows basic military protocols.
- Fighter Pilot: Knows basic and military flight protocols. Can land on a carrier, fly at night or in severe weather. Can fly any small plane. Can fly any aerospace fighter. Competent dogfighter.
- Astronaut: Can fly any space or trans-atmospheric boat or small ship. Knows spaceflight and re-entry protocols. Competent in zero-G and knows how to use a spacesuit.
- Mech Pilot: Knows basic and military mech protocols. Familiar with all Earth mechs; Competent with his specific assigned model.
Design Intent: What matters is "Can you do this under stress?" in terms of skill, so a binary proficiency system is a good idea. This employs, in a freeform fashion, Feats. It starts with Familiarity (i.e. can do it; 20 hours of training and practice), Competence (400 hours gets you a roll without incompetence penalties), and Mastery (10K hours; removes all possibility of fucking up on your end of the roll). In short: Can Do It, Can Get It Right, Can't Get It Wrong.
Binary conditions are also commonplace in the playable spaces: either you did the thing or you did not , you got hurt or you did not, etc. and there is no reason to fuck it up with mechanics that don't work this way. When relevant, removing or adding conditions (e.g. Mastery of your Mech means your attacks and saves can't auto-fail on a Natural 1, because you Can't Get It Wrong; you can only be bested by your opponent (i.e. lose the contested roll).)
Damage and Injury interacts by countering benefits or imposing penalties. (e.g. Enough damage to your Mech and you can auto-fail again until you swap or get it fixed.) Aside from the use of Saves, this is deliberately left unspecified because this is best handled at the table by the GM to make rulings on the spot according to the emergent events arising in play.
Now I'll have to clean this up if publication is the plan, but for home use this is just fine. Things that need to be cleaned up can be done at the table, and should be anyway in response to player feedback; that's just good playtesting protocol. However, this is a viable, playable Palladium clone. All you need to settle on from here is how to stat up the mechs themselves; I suggest using the existing Size categories as a basis, and introduce a basic Scale interaction rubric for when Dick tries to use anti-mech rockets on foot against Fabulous Fromage's mech.