Blueyonder, Good choice with that Hobbistar 60. It is a good flying, rugged, as you say old school trainer. I bought one back in about 2002 and still have it in my fleet. It gets flown only once or twice a year now but is still fun when it's turn to fly comes up. Mine has been converted to a taildragger and been reskined with a current Nexstar look. The original O.S. LA 65 still spins the prop and is really a good power match for the airframe. A Saito 100 or your O.S. 91 FX are way more than needed but that's what throttles are for. Either of those big engines will require a fair amount of weight added in the tail area to achieve C/G balance. My old taildragger one with the LA 65 needed 3/4 oz on the tail to balance, yours with a 91FX will need a little more. The small amount of extra weight for C/G balance won't be an issue in this plane, but the 91 FX really is more power than you will use and if you also have maybe a 61 FX or 65 AX, use them and save the 91 FX for a larger plane down the road. Good luck with this project with whatever engine you use. It will fly well.