There are no modifications required to upgrade the stock mainshaft bearing with a sealed one, aside from removing one of the bearing's seal, if you choose to do so. The stock bearing is a plain 6303 ball bearing and the one I installed is a 6303 2RS, meaning is has two rubber seals (one on each side). There is also a 6303 bearing with metal seals but I don't have any info. on whether or not one of those seals could be cleanly removed as I've never used one before.
You have two options. You can:
1. Install the bearing "as is", with both seals intact. The bearing comes packed with grease and should last a fairly long time. Just how long is up for debate since this depends on the quality of the bearing, how much abuse your gearbox is subjected to, etc!
Or,
2. Remove one of the seals and install the bearing with this open side facing the inside of the gearbox. I small o-ring removal tool or needle can be used to raise the lip of the seal to allow you to pry it off. The gear oil splashing around inside the gearbox during operation should keep the bearing well lubricated, and the remaining seal on the outside will keep the oil from leaking out past the bearing.
I found the stock bearing suprisingly easy to remove. I placed the largest washer I could fit on top of the rear of the bearing and tapped on this with a drift (you could use a large socket or extension if you have no other choice). A large socket can also be used to tap the new bearing into place. If you have to tap on the bearing itself, tap only on the outer race of the bearing and do so evenly around the bearing as you go. If you tap on the seal or the inner race you'll risk damaging the bearing!
The other end of the mainshaft also has a larger ball bearing but has an oil seal that is pressed into place over it. However, I notice that Hitchcock's have a sealed bearing upgrade available that contains a sealed bearing both for the front and rear bearings. I suppose this would offer an added layer of leak protection.
Anyone that has any input on this bearing, sealed or otherwise: I'd love to read it!
Concerning the 2mm vent hole I drilled in the inner cover: If I recall (I don't have the manual with me at the time of this typing) Mr. Snidal suggested drilling the hole maybe 1/4" (?) below the upper lip of the cover but this, by my estimation, would be within the depth of the filler plug's threads. So, I drilled the hole a little below where I determined the bottom of the filler plug would be. The "hump" that the vent hole is drilled into provides a little pocket on the inside of the cover that's perhaps just a little more "out of the way" of the oil inside the cover.
Good Luck!
Al