Help with 49cc mini bike

[jburton46]

Just got a little cheap bike for my cousin to learn on (see pic)

It needed new rings and a new pick up coil and rear shock.

iv got the new coil, trying to find a shock off an old mountain bike and the rings are ordered.

When i get the rings on how do i get the barrel back on without damaging the new rings ?

Thanks

http://www.eco-import-export.com/ima...-dirt-bike.jpg