My Gilera Runner (moped) is losing power and can not get faster than 20 mph

Helo,
the first thing you are going to do is clean the spark plug,or its better if you try to replaceit.,of you do that but still the problem is the same,then check and set the "timing" if yourt motorcycle is contact point operated,.
But if it is CDI operated,then you have to replace the said CDI.
If you have done all these things but it is still doesnt work good,try to check your ignition coil if it has loosen contacts or it is already defective.
goodluck..