Mecha and Personal combat is based on the premise of Skill + Stat + 1d10 + modifiers vs Difficulty(DC) or against another critter's own Skill + Stat + 1d10 + modifiers. If you rolled a nat 10, you rolled again, adding it, and probably bagging a critical success. You also scored an effective critical if you beat the target value by 10. With mecha, your machine's responsiveness and formfactor modifies your available stat value. A light and whippy scout? Like a second skin, maybe. A big chungus without anything to offset its mass? Might be worse than driving a tank.
Given its mid-80s/early 90s roots, it shows its age in that Reflexes is the stat for combat. Myself, I recommend using the character's Movement Allowance for Dodge, and Technical Ability for sniper-type shots.
Armor worked a little different from D&D derived systems. It's a Resistance-style value that can degrade, depending on how hard it's hit. Basic armor degrades by 1 per 1kill hit, advanced armors may require up to 8kill+ hits to degrade. You could also get some energy-absorber armor, but it was more brittle than more generalized armor.
Powerplants also had Explosion Saves(XS) based on if they were Hot(5) or Cold(1), where if you rolled under the respective value when called for it, it blew its stackpole. This value went up for each hit to the powerplant that it survived.
Then, there was the hit location chart. Regular hit? Figure out what segment got hit(Head, Torso, right & left arms, right & left legs, toss in wings, tails, pods, extra turrets(heads) as needed), as well as subassemblies on really good hits, and cinematic/catastrophic subassemblies & effects on certain criticals.
The main problem most players and referees/GMs will face will be in the subassemblies, which feature:
Weapons. You buy the basic damage in Kills, ranged weapons get a basic range for free. Modify base cost by desired range change, accuracy, rate of fire, special effects. Base total cost = raw space, 1/2 Damage Kills = tonnage. You can fiddle with these values for extra cost. Missiles are bought per each, boolitz by the space, and both of these have a variety of special effects, from paint rounds to thermonuclear cannon shells, and Hands are actually a Quick & Handy melee weapon. You can also do heat sabers, laserwhips, etc.
Variable Weapon Systems. A pool of construction points and a number of pre-sets that can allow you to save weight & space at the cost of having to use only one at a time. Remember the Might Morphin' Power Rangers' little pistol-dagger things? Or Iron Man's palm-blaster/thruster things? Those are VWSs. Each different attack or firing mode, plus the thruster mode, are a different entry in a VWS' portfolio.
Thrusters. Reserve 10% mass for remass/fuel, calculate space & cost off of total tonnage by propulsion type, or use drop-boosters and suffer sluggish controls.
Command Armors. Extra armor, spaces, in exchange for the aforementioned sluggish controls and added cost. See the chobham armor on that one RX-78 variant.
Sensors & suites(basic-ass sensors, radars, magnification, etc), little asides(stereo systems, anti-theft systems, firefighting equipment, etc), weird shit like Remotes(sub-mecha working as your flying fists, cruise missiles, suicide drones, etc), and shields, both of the chunk-o-armor variety(may even come with an armature to position it for you!), and the energy-based type.
Cost Multiplier Systems such as Internal Automation(AI-driven machines, or simply self-aware machines), different ratings and types of powerplants(ICE mechs, fission, fusion, exotics, etc,) Maneuver Verniers & Targeting Computers(offsets the tonnage and formfactor penalties), Combiners(good fucking luck, shit's a headache), self-healing or biotechnical machines, different control schemes(steampunk, exposed(motorcyclist-style), standard, plug-in, VR), and some other shit.
Formfactors. A cost multiplier system if you're making a transformable/variable machine, otherwise just make the listed adjustments to your design. Most impact your motive systems, hardpoints, and recommended servo locations, but a few also impact your armor or melee damage values. Most anything from bikes, cars, tanks, planes, gerwalks, beasts, submarines, ships, etc.
Stupid Mekton Tricks. Wanna go full pants on head? Try Ninja Jumping, Akira-style or grey goo absorbers, turning your robot into a camera or microscope(don't forget mass displacement, or do, you do you), or even building a robowaifu.
Scale. The default scale for any mecha construction is 1:1, or Mekton-scale. This is somewhere around the size of a Mobile Suit or Battlemech. Humans are 1/10th, most cars and pickups are 1/5th, many vessels and big aircraft in the 1:10 range, and starships in the 1:100 range. This is how you can build a dragon the size of a small mountain with this clunker of a system.
