Reinstalling the magneto involves the opposite steps to removal in reverse
order. If you need a fresh gasket for the mounting flange and do not have
one to hand,
for a pattern that may be of use to you.
In addition, for chain
driven magnetos, you will need to set the tension of the drive chain.
Lastly, you will need to set the ignition timing. Consult your
machine's service manual or the appropriate web forum for your machine. If
you do not have a timing disc and would like to make your own, you can
download a pattern here.