Is your LIMA a 2-bearing or single-bearing unit?
The Lister ST will accept SAE standardized housing adapters and drive hubs. These may be used to close-couple the pair and yields a very rigid and reliable machine assembly. This is generally only compatible with single-bearing generator heads.
If you want to use a flexible coupling and a rigid base to align and secure both the engine and generator head, then a simple shaft and lovejoy-type coupling can be used.
belt-drive always will entail extra maintenance as well as a bit of friction losses in the belts and pulleys.
The parts are fairly readily available as there were many thousands of these produced and used throughout the world.
dieselgman