RE: bobcat 25 and 50
The O.S. .55AX would be a good choice for the .52 bobcat.
Many are are also flying it with the Tower Hobbies .75 engine ( 2.2bHP ).
I'm flying mine with the GMS .76 ( 2.5 bHP ) ( current availability "late march" made by the same company that manufacturers the SuperTigres and T.H. engines ).
I would venture to say that the .75 or .76 may be a better match to make this a fast and powerful plane, not that the .55AX is a slouch either...
But to get the same performance levels out of the .55AX ( 1.75 bHP ) you need pipes, etc. so you have a somewhat greater expense for about the same or less performance.
Slap a Graupner 11x8 Pusher prop on the .75 or .76 and you have a great speed demon that scoffs at this thing called gravity.
The OS .32FX is the predecessor to the OS .32SX.
OS seems to have changed the liner material and the carb design in the SX, inproving both over the older .32FX.
The .32SX is getting a bit long in the tooth now though.
The bHP specs are identical for both.