Start by checking the fuel system: filters, fuel quality, fuel lines, etc. EFI engines are finicky when dealing with contaminated fuel. Look too through the wiring system and check for loose and corroded connections, check the battery condition. The idea is to eliminate the obvious. Try to find a good service manual for in-depth EFI electrical tests. Don't forget to go through the drive system as well, a bad bearing, bushing, track alignment can create significant drag issues that can affect engine performance. You are correct in wanting to keep the sled, I'd love to run that engine in a modern chassis.
