Engine on the right is a 31cc ryobi long shaft.I have a hub set already made up.I wont be using it either.One on the left not quite sure,i need a little more light on the case and the other side of engine.Some of us rebalance our flywheels some dont.Usally there not that far off anyway that ive come across.You only need about 3/8ths of an inch of threads to hold the hub on.The hubs like ya mentioned that we say are easy to make is because some of us have lathes,drill presses,ect...The long shaft ryobi for instance just uses a spacer to slip over crankshaft,about 1 inch in diameter,then you put the prop on,then a washer,then the nut.
To figure out the ccs,use a popsicle stick or similier.Insert in muffler port and touch other wall,then bump cylinder and itll leave a mark.Now measure this in MMS and write it down.
Use other side of stick,insert in spark plug hole,then mark at TDC and BDC.then measure the distance in MMS.
Lets say for cylinder you measured 30mm.
Then for stroke you measured 28=====top dead center=====bottom dead center
You want 30mm squared,your equation will look like this.
15x15x3.14[pie rounded]=706.5
706.5x28 stroke=19.78
This means actual ccs are 19.7,but the piece of equipment will say 20ccs or the parts listing.
If ya want C.I.=19.7x.061=1.20
This engine would be close to a 120.basiclly.