It's definitely fun. I built a 2 speed motor on a balboa like 10 years ago and absolutely loved it, imported engine had the 44mm "D" length crank and I built it up with the Athena 80 and some custom clutches etc, had it up actually over 60, eventually went back to stock D cylinder and it'd still get mid 40's easily. One of the longest I've had any bike. I still got it now.
I've built 2 more for myself since then, and sold or traded em or whatever, built a few for friends too. it's a very satisfying setup. This one's limited in that it still has the stock pipe and carb and the 42mm "A" crank, but a regear, 15 bing and any decent pipe should be a quick reliable 40mph bike. As-is it 's in the low-mid 30s and apart from the leaky carb (it's not airleaky, it leaks gas out) it's pretty reliable or at least it has been this past summer.
But they still suffer the sachs clutch woes, they're a small clutch. This one I'm pretty sure is just the regular 5 piece although I was trying to fit 7 in something last spring, maybe it was this one? I don't remember, been a few sachs this year.