I have a dedicated CMMG upper 22lr(wasp) and used the 22 mag conversion kit to use S&W mp22 mags and it works a lot better and also gives you a true lock open on last round. Plus m&p 22 mags are cheap and easy to come by
Lots of guys are swapping out the poly upper for an aluminum one. Seems all the specs are the same as an AR (barrel nut, shank, barrel diameter, etc.) and the bolt is just a standard .22 conversion style bolt.