G'day Harry
If you already have the metal needle just use that and see how it goes.
Yes once you replace the decompressor with a plug if the compression is down you will know the rings / cylinder are worn.
If the top of the plug is in the way of the top plastic engine cover when you block the decompressor hole I would just
break the top porcelain part of the plug off.
Another test is to start the mower and turn the engine off by turning the fuel tap off and just before the engine stops if it
runs a lot quicker you have an air leak.
(If the engine accelerates or runs more quickly just before it stops, it could indicate that there is an air leak somewhere)
Cheers
Max.
