I like engines that have good solid muffler mountings, ABC, AAC or ABN. I hate bench running engines, mostly because we don't have any place to do it except at the flying field...which we also don't have, currently. When at the flying field, I want to fly, not fiddle with engines. Absolutely don't want to fiddle with motors!   With an ABC, AAC or ABN, I usually mount the engine in the plane, run a tankful on the ground, and if it seems "stable", I'll fly it a bit rich, maybe with a small-ish prop. Works ok.

The ST .23, being ringed, has that against it. They're a pretty zippy engine, and should be fine for sport, but would need less prop pitch, to keep the speed down. Probably 4" or less. Ringmaster or Flite Streak!
 

If you're going to compare anything to anything, you need to start by comparing apples to apples -- i.e., the most powerful to the most powerful, the easiest-handling sport engine to the easiest-handling sport engine, etc.  Particularly if you're talking about stunt, you also have to decide if you're going to take the level of knowledge into account (the LA 46 would have been considered a terrible engine in the days when only a 2-4-2 run was to be considered, yet it's a really nice, inexpensive engine with a great 2-2 run).

Just comparing one random "old" engine to one random "new" engine with one random criterion doesn't say much at all.

I generally agree with the above observations. The ST G20/23 was a hot-rod, close kin to the most powerful 15s/2.5cc engines of the day, and legendary. The Brodak engine is a pretty inexpensive "throwback" engine.IF you want to compare apples to apples, try the STG20/23 to a NovaRossi 3.5, or even a FORA 15 FAI speed engine. But you know the answer already. Either the 25FP or new (another very low-ball sport engine once available for $49) or the "new" 25LA will likely eat the ST for breakfast. And in either case, raw RPM on 8-6 props has nothing to do with stunt performance.

    Note that your other engines do not provide "excellent stunt runs" where you define "excellent stunt run" in terms of the last 30 years. An "excellent stunt run" has generally been spinning a ~4" pitch prop at around 11,000-12,000 RPM with nearly infinite tunability, and at least a moderately effective muffler/pipe, usually with no "phase change", like a constant 4-stroke or a constant 2-stroke.  Your 40/50/60 STs and Foxes won't do that, I know, I used to use them too.

     If all you want to do is sport flying, then, maybe, but replacing a Fox 35 with a 20FP provides a drastic performance inprovement, in addition to being nice and quiet is dead-nuts reliable, has next to no vibration, and can be run on cheap fuel that you can get anywhere.

    You can do whatever you want, of course, and if you have serviceable engines, by all means use them. But don't let nostalgia cloud your perspective, a lot of this have been through that era and wound up where we are today for pretty good reasons.

     Brett

Ugo Rossi and Juares Garofalli have been in the engine bidness a long time.  Same-same OS.  The other engine-maker you named is a great business man, but not really an engine man.