Hi Ernst,
A paper gasket between the crankcase and the cylinder is a good idea, particularly when (like here) there is a joint between the crankcase halves, and they may be at slightly different heights.
There should be no noticeable step between the halves, particularly if the halves have the same "match" numbers. But if there is a step for some reason, it is better to fix it by machining rather than by fitting a thick gasket or some kind of "goop".
If you need goop, I use Loctite Aviation Gasket Sealant No. 3, which I like because it doesn't harden. You can smear it on a paper gasket for extra sealing.
Cheers
Leon