Shorten the 3 clutch pins... well you gotta do that or the new cover will most likely come into contact with the pins and things will get ground up badly.
Beg or borrow an angle grinder of you have to.
The shear pin in this case it the woodruff key. It is a half moon shaped piece of semi-soft metal. In case of sudden seizure of the engine or drive train the woodruff key is designed to shear and sacrifice itself before any damage occurs to the gear teeth or other engine parts. It also locks the gears to the shafts prevents the gears from slipping on the shafts.
"Machined away most of the taper"....... No 2 engines or clutches are identical and the tapers of the shafts as well as the diameter may or may not match the taper and diameter of the shaft hole in the hub of the clutch. Most all will come close enough to work, but not all. Sometimes the clutch will need a little "massaging" to fit properly.
"Machined a keyway".....
The Centrifugal clutch does not come with a keyway machined into it to allow the use of the woodruff key. Most of the time this results in the clutch slipping on the shaft and this can do serious damage to both surfaces involved, the clutch hub hole and crankshaft. Some folks machine a notch into the clutch hub to allow a custom made "L" key to fit so as to lock the hub and shaft together and prevent any slippage between the two.
silverbear has a good thread covering his experience with a centrifugal clutch and making a notch for the woodruff key.