JoXX, The OS FSR at its time was the O.S. top dawg! There were many clones. The Royals, and the first Asps plus others that escape me at the moment.
The Royals threw rods and crankcases rathe quickly. However those ASPs as imported by the late World Engines were and are excellent. Easy to start and handle, and just as powerful as the O.S. When World Engines passed on to Indy RC which was assumed by Horizon and the ASP was significantly changed, and I have no clues to whatever happened after that. I have several old ASPs, NIB. The secret to knowing the FSR is the bolt-on front crankcase.
I never followed O.S. much for reasons unknown.
However if they were like the ASP (O.S. Clone) the OS may need a couple headgaskets to help get a better CL type run.
Remember that the RC run is a tad different and probably current OS may have a different compression ratio than the FSR. Back in those days, anything over 10% nitro was
"High Nitro" and most engines were 6:1 to 10:1 ratios which worked well for general flying. Some of the engines today are - and were - back then, overly compressed to attain high power at little nitro. If you sock 15% in your FSR it may not like it very well. I use 5 to 10% for my old engines.
I remember that the FAI pylon and CL Speed were restricted to NO nitro and used 18-22 : 1 compression ratios. Glenn Lee, an old timer in CL Speed could turn 28,000 rpm with 80% methonal and 20% castor oil. I couldn't get 20,000 with 70% nitro. [&o] OTOH I blew some crankshafts on FR engines.
See you around when you get those machines going.