I do not think there is anything in the "rules" that prohibit modifications to the engine(s) used to go for the world speed record for piston powered aircraft. The Bearcat that holds the record was powered by a British Centaurus engine, far different than the P&W 2800 originally in the Bearcat.
Keith
FAI Record File Num #8437
Status: ratified - retired by changes of the sporting code
Region: World
Class: C (Powered Aeroplanes)
Sub-Class: C-1 (Landplanes)
Category: Not applicable
Group: 1 : internal combustion engine
Type of record: Speed over a 3 km course
Performance: 850.24 km/h
Date: 1989-08-21
Course/Location: Las Vegas, NM (USA)
Claimant Lyle Shelton (USA)
Grumman F-8F "Bearcat" (N7771)
Engine: 1 Wright R-3350 Duplex cyclone