Downunder, I'm over in the US but if you want any of your engines worked on, I pay for return postage. You can get replacement gaskets, look for gaskets for a SH .21, SportWerks Mach .21. they are the same engines. I could probably find gaskets over here than you could in australia, just because RC stuff is cheaper and more plentiful here.
Generally, when you tear down an engine, you will either pinch or replace the piston sleeve assembly. Pinching consists of taking the sleeve and slightly squeezing the top of it to create a tighter "pinch" between the piston/sleeve.
The easiest and cheapest method to get more "pinch" out of the motor is to take a pipe cutter (a copper cutter like you would find at a hardware store) and run the blades around the lower oil ring on the piston body a few times, and then test it in the sleeve. If you do it right, it will recompress the engine and prolong the life. I have "cut" my JP stock motors 4-5 times before they were done for and I have been able to get 14-15 gallons of consistent power from a "cut" motor.
Other than that, the only thing I would check is the crank bearings. They leak from time to time, and need to be replaced.