I know a couple people have done serpent conversions, so it might be worth having a search for them to get some ideas. The Quark is a solid 5s esc, but the heat issues are a pain in the butt to fix. Price wise, there isnt much between the MGM and 125b, but you will need the programming cables or the card to make life easier with the MGM. As for bigger .32 pinions, 22tooth 1/8 bore is as big as you will find really without getting some custom made:
https://sdp-si.com/eStore/ Click on Gears > spur gears.
Mike sells some fairly big .32p 5mm bore pinions if you go with a larger feigao or neu etc.