You are faced with a number of options here. Any of the CM/CB 400/450 engines is a bolt in operation pretty much, depending on the actual engine there may be a couple of little things to do.
Advantages of the 450 besides the obvious 50cc's are the oil cooler, 6 speed trans
(OD 6th), solid balancers and better cam oiling. Downside is IMO a weaker oiling system.
Advantage of the 400 is a better basic oiling system. Downside is 3 piece balancers, weaker cam oiling, no oil cooler.
Might wander thru this topic viewtopic.php?f=58&t=19761&hilit=79+CM400T+with
There are parts available, just have to look for them. Some sources: http://www.davidsilverspares.com/parts/by-model/
http://www.servicehondapsn.com/fiche_se ... &mfg=Honda
and this ebay seller has pistons http://www.ebay.com/itm/HONDA-CB400T-CB ... 1268wt_663