FWIW my understanding is only the 16 engine will fit the MC16 frame as it is rigid mounted unlike later models. I have both a 16 and a 21 but haven't tried to swap a 21 engine in. To upgrade the 16 engine, fitting MC18 bits apparently is the way to go.

Best thing you can do to learn more is get a manual from Charles here on the forum. Great source of info along with the parts manual on this site.

Tyga are a good source for consumable parts. You can use later model pistons, rings... in a 16 engine. Japan is about the only reliable source for other 16 parts tho check ebay UK ad ebay australia. Parts come up there regularly. David Silver Spares in the UK can also provide parts such as seals and M/C or calipher rebuild kits. Learn to repair plastics as 16 plastics are hard to find!