If possible turn down a piece of bar with a spark plug threaded end and a (say) 9mm hole running through it to substitute the plug. Take an 8mm fully threaded bolt of suitable length and round off the end of the shank. Slide an electrical terminal under the bolt head and secure with a nut. Place an insulating fibre washer under the nut and then Insulate the shank with heatshrink or similar so that 1, the fibre washer is held in place, 2, the bolt is still free to slide inside the bar and 3, the tip of the bolt is exposed. Put the bar into the cylinder head and insert the bolt. Ensure the bolt is free to slide up and down as it contacts the piston. Connect an electrical multi meter to the leccy terminal and an earth point on the engine. Attach the timing disc as usual. With light finger pressure on the bolt head, rotate the engine until the piston contacts the rounded shank in the cylinder. The multimeter will give off a continuous beep as the circuit is made. Note the before TDC figure. Keep rotating the engine (bolt extends out and then back in) until the beeping stops (piston no longer in contact with the bolt). Note the after TDC figure. Halve the difference and turn the disc one way or the other by that amount. After a couple of goes you should narrow down the position of the timing disc so that the beep starts and stops at the same number of degrees either side of TDC. It is possible to get that piston to TDC exactly or at least to within half a degree.