Most Compression testers aren't accurate ,50 psi is the lowest I'd use with my comp tester as I know the real reading will be at least 65 psi and have used mowers like this for 10 years and haven't had a problem.
here is a video showing how far out different comp testers can be.
I'd go and fix the carby and use the engine with the higher comp.
You can run a mower on 65 psi ,below is a LawnBoy motor spec that has minimum 65 psi cold.
If the Schrader valve isn't at the spark plug end on the compression tester the flexible hose just adds to the compression chamber volume ,thus giving a lower reading.
