Lister made a boatload of different 2 cylinder diesels thus your information is too sketchy for anyone to help other than pure guessing. A picture would help ID it but that's not going to give you a specific RPM as the engines could be spec'ed at varying RPMs. Chinese photo tachs are cheap, buy one and check it running.

Yeah, I gathered that much, although I am not sure whether here in South Africa we were privy to receiving a lot of Listers. If you send me your email addy, I can drop you a couple of photos, while I in the meantime get a tacho