I used to have a timing tool for motorcycles (before I had a dial indicator, when timing was spec'd in mm before top dead center) that I made from an old spark plug. Took the ceramic and the center electrode out of the plug, slathered a chunk of copper tube in JB Weld or epoxy and stuffed it in the spark plug body. Free hose fitting for the spark plug hole.
The timing trick involved getting the piston above the exhaust port, pouring in oil, and using the engine to pump oil up a long glass tube of known inside diameter. One mm of piston movement made hundreds of mm of movement in the oil column in the tube.
Simple thing to do would be to make a spark plug hose fitting like I described above, block off the intake and exhaust ports, and run the piston down below the transfer ports so the area below the piston is open to the area above the piston.
