Slipping Centrifugal Clutch

Those clutches are not keyed to the crank shaft.
Silverbear went through what you are going through.
http://motorbicycling.com/f34/centrifugal-clutch-blues-17162.html

You can convert the engine back to mechanical clutch, but you will need to purchase a new gearbox cover, crankshaft gear, and key.

WAIT !,,,,,The flower nut may need adjusting,the motors I've seen on here still have the clutch arm and it is used,meaning the regular clutch pucks may be slippingIf its been slipping more and more over a 40 -80 mile time frame then I would try the flower nut adjust first(sorry ,I know the centrivical clutch has to come off to adj.the flower nut)

Very true, corgi1!
My mind just went back to the most recent thread concerning this exact problem.
For saetta's sake. I hope that it is as easy as a simple flower nut adjustment.
Now I wonder how tight his clutch cable is when in the engaged position.

As for doing a conversion back to a manual clutch, yes, the parts should interchange.
You will have to be careful that the gear teeth mesh correctly though. I doubt that they will fit together too tightly, but they may fit too loose. They need to overlap each other by at least 75% for good power transfer. If the peaks of one gear go all the way down into the valleys of the other gear too tightly it will cause a horrible whining, moreso than they normally whine, which is alot, and cause excessive wear.
Try corgi1's suggestion first.