A low compression feel is usually normal for four-stroke engines, but if you flip them backwards (no glow plug battery attached), they should have a good compression feel. If your engine has zero compression feel when hand-flipping it in any direction, then you have a leaking piston ring or leaking valves. If a bit of fuel squirted into the cylinder brings the compression, then you may be OK.

The engine will run when it's spun fast enough because the piston is moving too fast for the fuel/air mixture inside to get out, and the resulting compression will push the ring against the cylinder wall, and will push the valves closed. The firing process will also do this.

If the engine runs well, has a good idle and throttle up/down response, just fly and enjoy the engine. When the idle starts to become unreliable, then it's time to rebuild.