Update: Just installing an Centrifugal Clutch on the Mill Engine. [Acts like an Automatic Transmission.]
The engine had an 1 1/8" output shaft, I had Clutch with a 1" Bore. I ran the engine and with sandpaper, file and micrometer I took a little over 65thousands of and inch off the shaft and milled a key with a step in to the size of the new key way. drilled a hole in the key to aid in removing it if needed.
In the first picture should be able to see the key with the step and the hole to help removal. In the second other the key driven in to position. A 7/16" X 20 bolt with a large washer will hold the clutch in place.
Saw this old thread and thought I would chime in. Portable sawmills are a great investment- we can testify. It's half paid for itself within this first year we have had it- we've sold lumber for orders and even sold scrap wood. Our cabin has been built entirely from wood we have milled ourselves. It's hard work, but we have enjoyed it. (Assembly was time consuming and a bit frustrating.)
Check out the photo below of some cedar we cut for an order that they turned into a door!