Instead of fabricating a new bearing retainer, why not simply machine the old one to accept a seal? I'd be tempted to clean up the OD of the input shaft to get it round, and press fit a thin machined ring onto the shaft to act as an inner seal surface, also. Bearing and seal manufacturers such as National, Chicago Rawhide, and SKF publish progressive size charts for seals that list the ID and OD, as well as the lip material and design. Just pick one that meets your needs. Since the SM 420 has survived for decades without a front seal, not all modifications are "improvements"- - - -especially if you're not going to encounter extremes of up and downgrades like the rock crawlers do that might let the lube leak out of the front of the housing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!