Peter
I guess it depends on how much money / hassle you want to go through to keep an Autocrat flying.
Does it really need a new engine ?
A while back now I did see quite a few parts, including a minor engine for sale in an Australian magazine / web site. I think it's called 'rag and tube'.
Or you could try and get something out of the man at Carr farm (Cliff baker ?)who has a whole collection of austers, but doesn't like talking to people
Other than than you could convert it to a N model with a major lump and parts. £££££££