I'm obviously a new member and all, and wanted to help a situation that affected me too.
So I noticed these threads have a lack of success so far finding viable trainers for MekTek's 3.1 version of Mechwarrior 4.
In these threads, I did find a weird Russian trainer for v.0.27 and with a little research, concocted a trainer that is so far, working with my copy of 3.1 MekTek MW4 Mercenaries. The only option it has is no Weight Limit, but that's the only one important to me at this time.
With a little more research I could probably add more options too it, but I'm sure someone with more experience with the Cheat Engine program could do it a lot easier than I could. Link
So there it is, it's ugly, the readme is crappy, but it works for me, and I'm running Win 7 x64 which is not so compatible so it should be good for others as well.