You can improve most, of the new or used or old engines, FOX 35 for example run much better, and smoother with a balanced or new CNC crank. It will produce more usable power and last longer because it is much much better balanced.
You can greatly improve the life of it with a cnc stuffer backplate
You can improve the run quality, and a slight power increase with a Smith hemi head
You can improve the needling a whopping amount with a PA type NVA , or even a OS 20 nva with aluminum shims to center it wworks better, other drilled out the case venturie bore and used the larger .157 OD Super Tigre NVAs
You can improve the balance, run, power, and make the engine last longer with a good AAC, ABC, or ceramic P/S setup, plus they will give a large boost in power, and even using the stock crank they will be in better balance.
I have seen FOX 35s pick up near 1000 RPM increase with WAY smoother runs using aftermarket parts, they also will last much longer
There are also many thing you can do to HP 40s, ST 46s, ST 51, ST 60s , to make these much better engines,
You can greatly improve many OS engines, as well as OPS , Magnum, and Thunder Tiger engines
But you need to know why what and how to do these mods, many 1000s of engines have been killed, damaged, or wimpified/woosiesfied
by doing wrong things to them
Randy