Well, I will say that the Micro GB is proving to be a little bit weak and that front end tends to be under a lot of stress, so that's sort of a known design flaw at this point. (You'd have think they would have learned after the Version 2, but anyway...)
It is the same gearbox as the Vz. 61 and Mac-10, so those two guns can also serve as a source of parts. (note that the Vz.61 is rear wired, not front wired, but they are the same otherwise.) Bad news is that no third-party company has released one yet, and it's very expensive direct from Marui. (About 8600 yen, I believe.)
My google-fu did turn up a site called www.airrsoftgunrepair.com
that claims to sell replacement GBs for $30, but I cannot vouch for how legitimate or reliable it may be. That was the only source I could turn up of the Micro GB.