I presume yours is a Bultaco version as opposed to a genuine Rickman version? I'm not hip to all things Bultaco but I have owned one of these little waterpipe wonders and have been through the process myself but sold it before I fitted a Triumph engine after failing to locate a proper Bully motor. 4 speed Bultaco stuff is getting pretty hard to find but I've been told a good base to build from is a Campero or early Sherpa S 200cc engine or something similar. You'll have more luck finding a 4sp 250 in the US than here or SA but you never know. What about placing an ad on Mark Swapmeet? It's worked for me in the past so you never know what might show up. Have you done some detective work to try and locate its original engine? Do some asking around in the area the bike came from, it may still be laying in a shed.