I have been toying with the idea of doing this myself after I have finished my 21. I think it can be done with some serious re engineering of the frame, apart from getting the engine in and aligned I think one of the big problems is routing the rear exhaust as fonty said. the r/h upper shock mount is bang in the way and needs modifying in some way.I have seen a pic of an mc21 with an rg500 engine in it and it looks like the guy has cut away the top of the mount for clearance and welded a brace on to put the strength back, plus you will need to loose the sub frames r/h lower rail. the other problem could be clearance for the carbs between the fwd frame rails.
An rg500 motor in an rgv250 chassis is probably a more straight forward installation and you can buy all the parts from here www.suzuki-rg500.com .
Or a rd500 motor in areverse cylinder tzr chassis like this www.nkracing.co.uk