the only real masters of repair that I know of are Mang Raul Quesada and LIto Bote (tolits).
While I also accept repairs, I'm slow and cumbersome compared to them. I usually am able to fix peds if the parts are available.
Modding, we have a few here on the forum. Of course my first mention is me (hahehehe) then there's (in no particular order) markv, Hardcore misery, jmorrero, backbauer and ricabb7. Maybe there are a couple more than I'm not aware of.