I am currently working on building a Z50R for my now 4 year old son. It is an 88 and pretty ratty at the moment, but it does run and all the parts are there. He either wants it yellow or blue (at least this week) and I am sure his taste in paint jobs is a bit different than mine or the rest of the purists here.
I think my son would fall in love with that bike.
As far as the chinese repro motor, I am also working on a trx70 for my son. It is cherry at the moment, but this winter it is going to get a 50cc, electric start, lifan, even though the stock honda is low miles and in perfect shape.
Two reasons.
One, He is going to learn how to ride on it so if he screws up the motor or tranny, I will still have to good Honda base for when he gets a bit bigger.
Second, he is not strong enough to pull start it, so the electric start will get him riding sooner
Good save on yours.