Replacement Outboard Motor

The Mariner 4hp outboard I bought with Speedwell, brand new in 2000, seems to be nearing the end of its life. Can any Romilly owner tell me what other engines will fit the original GRP plug? My Mariner is a standard shaft engine, and I have to remove its prop to get it to fit into the plug at the start and finish of each season. What should I buy to replace my engine, and what should I avoid?

Any suggestions would be much appreciated!

David Collin