There is a discussion thread on a 35mm project for ‘The Keep’ here, mate:-

and a couple of other non-35mm projects for it - 1 a Fan Edit, and 1 other Preservation project on it which may be of interest, or contain some useful info.

Good luck with it 😃

Edit: as ^…^ says in the linked thread above, there is also a thread on a 35mm project re The Keep on fanres - they’re a pretty cool, knowledgeable & helpful community - so may well be worth checking it out mate.