ORIGINAL: dszabo2
There is a piston locking pin you can get at any LHS that sells nitro cars. It designed to lock the crank shaft. You simply remove the glow plug and screw in the pin and it will keep the piston from going beyond TDC.
I think you'll also find the LHS sells new piston/liner sets after you punch a hole in the piston or cave in the crown
What some heli guys use is a tool that fits inside the crankcase (after you take off the back plate!) and has a groove cut in it to locate around the rod/crankpin. A bit more techo than the bit of wood I use sometimes but will do the job.