Re: 1987 force bad cylinder
If it has 0 compression, chances are reasonably certain that it has scored the cylinder wall. This requires a complete teardown and boring the cylinder oversized. You can replace only one piston, however if you read other posts you will note that there is some difference of opinion about this.
My take on it is: if money is really tight, just replace the one piston. However, it takes the same amount of work to disassemble and reassemble for 4 pistons as it does for one. So, If about an extra 300 in parts won't hurt you, might as well rebore and do all four. If you do the work yourself, replacing all four pistons will cost less than 1,000 bucks--probably around 750 or so. 500 for pistons and bearing kits, 50 bucks per cylinder to bore, and 50-60 bucks for gaskets.
Relatively simple job. Mostly hand tools needed with the only specialty tools being a press to push out wrist pins and a flywheel puller. Get a Clymers manual and a digital camera. Take photos during disassembly to refer to during assembly.
NOW: THE OTHER OPTION---If you are lucky (not likely) and the cylinder is not scored and the piston only has a small hole in the crown, then you can replace that piston only. You remove the head and the top carb and manifold. You unbolt the rod big end through the opening and push the old piston out. Collect all 16 roller bearings. Push in the new piston and insert the rollers then cap the rod. Done! Couple of hours max.
After repair, pay special attention to the carbs. A melted piston is almost always due to a lean run condition. Use the manual: be sure the carbs are clean, a filter is between the fuel pump and carbs, and the low speed adjusting needles are not too lean. 4 cylinder engines seem to like 1 turn out from lightly seated. Never-Never-Never go leaner than 3/4 turn out from lightly seated.
