The feeling of lack of compression is normal, especially if your prior experience was with a TH engine.

By comparison the TH engines are difficult to turn over when new and only loosen up when they have been fully broken in.

The O.S. on the other hand feel "light" initially and easy to turn over, almost as if something is wrong. Don't worry about this.

The low speed come somewhat set usually (most of my new O.S. engines turn right over on the first few tries after they get fuel.).

It is likely that the previous owner couldn't get the engine started, so he started playing with the carb settings and promptly screwed things up for you.

The O.S. is actually MORE tolerant than the Tower engines, but the engine you have is probably WAAAY out of tune given it's history.

You have to get the high end within reason, then adjust the low end (pinch test time) then re-adjust the high end again and double check.

Also remember to turn the cylinder head down once you have it tune to check for changes in attitude causing the engine to stall or lean out.

When it is properly set up the O.S. engines seem to BARELY change when they go nose up. Far less than the Tower engines, maybe due to more muffler pressure(?)...

It sounds like you are getting there, but the engine is probably still not in tune. Run a few tanks of fuel through it too to get it broken in.

As you do this change and check the settings at different attitudes.

Once you have the low end dialed in right, everything else becomes easy IMHO.