More details?
I think one of the newer SM420's I have already has a front seal in the front bearing retainer. While an older one I was surprised to find did not.
Can you just swap to a bearing retainer and possibly input shaft from a newer one?
Or, whatever they can do to it I bet you can too.
Do you even need one? I ran the older one without seal for a long while, still no problems and still a dry clutch.
Grigg