It may be quite difficult to find an engine case.
I know Tom @ Foundry Motorcycles.
He cast his own parts, melts aluminium in a fournace, makes sand molds, ...
NOT SURE AT ALL HE WANTS to be involved into that but you may give him a call.
On the home page of his web site there is a video where he cast headlights.
https://www.foundrymotorcycle.co.uk/
