Thank you all for the really useful information.
I had thought about a rebuild but a lengthy article on Morgans Cloud convinced me that repowering is the better option. The overall conclusion was that a rebuild can very easily go wrong so that you end up repowering anyway.
With regard to the Beta engine, a lot of people seem to shy away from turbo models. I can see that the 85 is a turbo. Is there any reason to avoid turbos? The Perkins is 30 years old and is a turbo and it seems to be just fine.
Next problem is finding a suitable engineer in Southern Spain.
Mark & Nicky Barter