use a ring compressor

These are quite inexpensive for a good brand.
You can also use a big hose clamp, but that adds some friction.

the ring end gap is fairly important, but if your just using std rings
in a std bore, you have no problems.

Maybe. Some rings REQUIRE fitting and are purposely made oversize

The old rule of thumb for automotive engines was .002" per inch of bore.
Might not be valid for an air-cooled engine.
If no specs are available from the Mfg or the ring maker, look for
similar engines with similar bore and ring size, from the same era, and
see what their end-gap was.