2 options I’ve seen used:
A) Treat it as a special equipment item using the rules in Ch 9 ( https://openlegendrpg.com/core-rules/09-special-equipment ). i.e. it gives the user STR 6, constant Resistance +6, and the ability to Fly at PL5 without rolling for activation. That particular model is expensive as heck, but cheaper suits are easily doable and the high-spec models give the players something to work towards, either by looting the entire suit, cobbling stuff together (which requires buy-in from the DM on which bits of scrap give which properties, though it also gives a lot of breadth for quests and quest-givers), or building up enough of a warchest to buy the thing honestly. This is the method that OL uses in Amauria’s Dawn; (at least) one of the NPCs has a power suit that gives them serious combat buffs.
B) Treat it as a set of feats, and make it so that only the character who takes those feats can operate a particular suit/mech. I’ve got a tinker character in my campaign with Alternate Form who has a suit of mechanized body armor (think the Marine suits in Starcraft) teleport in Iron-Man style, and he hops in it when combat starts. The tinker is physically weak but mentally sharp, with primarily crafting feats. The body armor is physically tough and loaded out with magic items that give it “ammo” to burn, but it requires all of the tinker’s attention to pilot it so he’s not really able to think at all quickly when he’s in the pilot’s seat.
You can do similar things without the Alternate Form, and just assume that the pilot is always in the suit, but that means that the player is going to be hesitant to get out and socialize/adventure in just their own skin, because they’re missing a big part of their character. If you’re running a mech-only game, though, that might be the entire point - if all of your players are weakened without their mechs, then it becomes a question of two settings for your game, one when they’re suited up, and the other when they’re not.
(Another way to achieve the same result of an everybody-has-a-mech game is to just state, flat out, that everybody has two character sheets - one for the mech, and one for the pilot, and build some sort of interface between the two so that particularly fast or smart or strong pilots gain an appropriate bonus while in their mech. Haven’t thought deeply about that option, though, so ymmv.
Maybe ban the mechs from having social or mental stats, and only let them have physical or extraordinary ones? i.e. your mech can be a melee brawler, have explosive missiles, ECM, or whatever, as long as its point buys adds up to the others. And then the pilots have to have a minimum total physical stat point buy, to meet the minimum physical qualifications for pilot school, as well as preventing the players from dumping their physical stats. idk, somebody with better game design skills than me needs to take a crack at that and see what breaks.)
Either way, feats make it easier to level up the characters’ combat abilities without needing to grab more wealth and without petitioning the DM to allow possibly-overpowered gear drops or additions. Since I’m running a fantasy game and the tinker has built an arcano-punk suit, I decided that he’d have to earn EXP to improve his mech (since there are no other mechs to steal from, or mech-engineers to buy from), take the feat tax for Alternate Form, and mandated that he actually has to act distracted and less intelligent than usual while piloting the thing, and left it at that.
Hope this helps. The system is so flexible that it sounds perfect for use in a mech-centric game. I look forward to hearing others’ suggestions.