Hi jason
It will be much more easier a restoration on a MC21 or MC28 cause Honda supply the stock parts compare to the MC16-MC18 discontinued.
The only problem on restorations of the MC21-MC28 models is that the most of the HRC parts are discontinued but if you want to rebuild a stock engine it is no problem.
If you save your money from the MC16 rebuild and buy a MC21 you will have many advantages first the stock parts are not discontinued,the bike is much nicer,with better performance and it is very common which this will help you in a restore or every repair to find some used or new parts.
Finally it is in your choice what to do,but think about all the parameters.