If you plan to take the lower part of the cylinder off, you should read <a href="http://www.mopedarmy.com/forums/discuss/read.php?f=1&i=9931&t=9931">Fred's guide</a> about putting it back on.
The first time I had to put the cylinder back on while compressing the rings, holding the piston, and sliding the cylinder it took me a while! It gets easier everytime you do it, though.
Although it can be tricky, it isn't very complicated. And as long as you don't get dirt inside the engine it's a pretty safe operation to perform.