Re bearings ... Presumably this is an open bearing without seals, and the output shaft seal is an entirely separate part.
Oil (pumped or splashed into and around the bearing) is perfectly fine for lubricating ball bearings and is the recommended lubricant at high speed. Grease is recommended for applications where the bearing is sealed (think: wheel bearings) and there may be a lower limiting speed if grease is used. Translation: it won't be a problem.
At least KTM had the sense to use an off the shelf unmodified bearing in that application ... no special seals, no special locating dowels or grooves, no non-standard dimensions. They discontinued it because you can get a replacement anywhere.
There's one other thing you could do, if you have a bit of wiggle room around the outside of that bearing once it's installed. Make up your own locating tab out of a piece of steel sheet that projects out over the edge of the bearing to stop it from coming out. Won't stop the bearing from drifting inward, but I suspect that the shaft that the bearing is locating will have a step on it which will stop that happening. Given the flimsy nature of the OEM locating device (round washer), you don't need much.
There's one other thing you could do, if you have a bit of wiggle room around the outside of that bearing once it's installed. Make up your own locating tab out of a piece of steel sheet that projects out over the edge of the bearing to stop it from coming out. Won't stop the bearing from drifting inward, but I suspect that the shaft that the bearing is locating will have a step on it which will stop that happening. Given the flimsy nature of the OEM locating device (round washer), you don't need much.
I am confused.
The "made is poland" 6004N bearing is the one you took out or the one you're trying to install?
A "N" suffix on a bearing denotes a snap ring groove.
If you bought a 6004 bearing, it won't have a snap ring groove.
... and no, you're going to cut a snap ring groove into an assembled bearing.
... if the shaft that goes in that bearing is located in the case on the other end with a press fit bearing and a snap ring, you should be fine locating the 6004 bearing with that tab set up, as the shaft "floats" on the bearing.