Thanks for your answer and your suggestions, Wacio.
I checked both the head gasket to see if there was any air escaping between the cylinder cover and the cylinder head: nothing. I checked on my valves and they're in good shape. I tight the input of my compression tester as I should've (according to it's owners manual) when I tested the compression; both my cylinder and cylinder head has no cracks. The only thing I can think about is either a worn piston and cylinder; because I changed my piston rings for a brand new ones about a month ago, and I still had the compression issue.
About the broken starter clutch, I had no idea that it'd be a possible cause of low compression. Do you have any idea about how to check if I have something to resolve there?
I think you're right on the possible consequences on upgrading the cylinder to a 300cc one. So, I think I'm not gonna do it; But i already ordered a big bore kit of 250cc. So, I hope that installing it and checking my valves clearances would be enough to resolve my compression thing
