Hi Johnkpap,
Have you examined the liners/pistons of both engines disassembled? Just curious as to what the porting changes were (photos would be great). I'm guessing there were changes to the exhaust timing and possibly/probably the intake timing. How about the crankshaft - any changes there? If there are changes to the intake ports on the liner were changes made to the engine casting?
Performance Specialties (Dave Shadel) sold replacement piston/sleeve sets for the OS 61VF back in the '80s (they were manufactured by Henry Nelson). The chrome plating and honing process (especially the former) are VERY specialized and only a few people (modelers) world-wide have the technical capability to do it correctly. If you have a Shadel/Nelson piston/liner then you have some top quality replacement parts.
My stock OS 61VF with 7D carb (with insert and no pump) was plenty of power for my Arrow. The pump is unnecessary in the Arrow as the tank has to be located behind the firewall - not much room in the fuselage with the pipe tunnel floor.
Thanks for the information and good luck! I'm looking forward to the flight reports!
-Will B.