Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

That engine certainly appears to be in very nice condition overall.

Those German engines from back in the day commonly had that style of sight glass, and in general it worked well for them. I think your insistence on using some form of modern goop is perhaps not really the best for getting the seal you are after. Most any pliable material that has the proper sized hole, like leather, rubber and some kinds of fiber washers, are likely to squish in as they get flattened by the cover screws and form the seal that you need. These engines are not operating under very much pressure at all, so a super seal isn't really called for ..... at least in my opinion, but YMMV!