From my friend NavidA1, the creator of Reloaded;

"As others have mentioned, top speed comes from hard values coming from mechstats table.
If you implement a mechlab and have a mech equip an engine of rating X, then you need to modify the parent class responsible for spawning mechs to update the speed according to your new situation (a custom loadout for example). It's not just the speed. You can modify every stats (assuming it is exposed) of a mech in assets like Derivedmech when the mech is spawned and activated.
I recommend creating a general function library including functions like "HasEngine" (custom name), which receives a loadout object and with outputs like yes/no flag, rating of the engine, etc, so you can use them everywhere in your mod."

I am currently working with a friend to make a somewhat similar but more free Mechlab. As it stands, we will be working on it the following way;

* v1.0 - Size-agnostic hardpoints
* v1.1 - Motive System Upgrade, BAP, and ECM changes
* v1.2 - Structure + Armor changes
* v1.3 - Mech Engine changes

I'll be sure to try and update our progress as we come along, though it will be tedious. I took a look at all the work Navid did... it's tremendous. I, however legit want to work on this mod even if I have to force myself simply because of how awful the Vanilla mechlab is, which is the main issue why my review of this game is so negative sadly.