A tip from the Greenspun Archives:
I just put my bike back together without proper clamps. I made some clamps out of a 1 inch wide strip of aluminum flashing. Heres how to do it: Cut a strip of flashing about 2 inches longer than the circumference of the piston, cut half way across the flashing about one inch from the end, cut half way across the strip on the other side about an inch from the other end. You can then wrap it around the piston and squeezing tightly align the slots and push then into one another. Getting the distance between the slots right is the key, once you have done one use it as a template to make the other 3. I put all the pistons half way up and fit the strips around the pistons, they should be quite tight and hard to fit. Fit them so they are level and overlap the top ring a few mm. Tilt the engine forward and carefully lower the cylinder onto the engine, once you have the top of the pistons in the holes, start to gently tap the cylinder and it should move down over the pistons. You may have to adjust the strips a bit to stop them going into the bores, once the cylinder is past the rings you can remove the strips by tearing off the ends and they'll pop off. Worked for me. It would work even better if you could fold over the top of the strips to made it wider so it would have less tendancy to go into the bore. You could use aluminum tin cans instead of flashing. Be very careful a ring doesn't pop out before its in the cylinder, if it does you can poke it in with a pointed bit of plastic. Make sure your cam chain tensioner and chain are in position first, I didn't and had to do it twice. I've used hose clamps and tin can strips before.
--Glenn