RE: A good .61 engine
TT 61 Pro is my choice for a 61 sport engine. Lots of zip, dead reliable, easy to tune & a very long life.
Forget the GMS
The Tower is OK -- really strong.
ST is OK as well, but the carb is so-so.
If you want a high quality, but inexpensive 61, the TT 61 GP is an excellent choice -- more powerfull than some BB types (OS 61 FX for example) & has the advantage of a long-lived trouble-free plain bearing.