Yeah, personally, I wouldn't change a trigger on one even if I could. If it came to that, I'd go with a dedicated .22 upper and use it with a lower that already has the trigger I want in it. That said here's a thread with some proven aftermarket triggers, maybe it can help the guy:
http://snipershide.com/forum/ubbthre...Number=2690274
I think I'm going to go with a Magpul edition M&P 15-22 and have a gunsmith re-profile the barrel to pencil spec. (I want a super light gun for my daughter to run in club matches.)