The old one will run and operate like it's big brother, the ST.60, but does not make as much power. There are a fair number of them out there, so parts cows can be had to keep one operating. Both R/C and C/L versions were the same except for car and venturi. The newer G version came in an R/C version and a C/L version, with the C/L version having a different cylinder liner with a bit milder timing. I have operated the C/L version a lot since it came out about 1994, to the extent that I'll guess I have several thousand flights with them in both stock and "reworked" editions. And by "reworked" I mean that they were just optimized for fit and details, by both Tom Lay and Randy Smith. It is a direct replacement for the OLD ST.60, of which I have many and fly them also, but it does not fit the same motor mounts. (After typing that, I do have to say that I have not tried to put one in place of another, but I believe I have measured the cases to know they are different.) The engine is not in production any more but parts are still out there as are plenty of core engines for basic parts. They are fairly light for their displacement and pretty reliable. If you search out on the forum you will find threads where guys have put the old ones in big planes like the Top Flite Score and other .60 size ARFs with not many complaints and you just need to watch the weight of any airplane you put it in. I have flown my newer G-51's in models as heavy as 72 ounces. With both being ring engines, you need to feed the accordingly with fuel that has castor oil in it, but a 50-50 mix of castor/synthetic is probably best in my opinion with total oil in the 20 to 22% range and 10% nitro the most I have ever run. I would give the newer G-51 the edge on power, as it will pull like a .60 but I could never get mine to swing a 13" prop. The prop prop of choice for me on the newer one is the APC-12.25-3.75. Old school props like a 12-5 or 12-6 will suit the old model. I think you would like either engine when you decide to use them.
Type at you later,
Dan McEntee