When it comes to buying non-OEM bearings unless I can find a seller with ALL the bearings the price of postage would drive up the overall price (I'm in Ireland and a lot of these sellers are in the US with higher postage rates than bearing cost).
The main problem for me is the number of different type of the same bearings out there. Have a look at: Koyo
I understand (in a general sense anyway) the C3, C4 etc. but not sure of the rest of them and how to get an exact match to the original Yamaha part.
So, I think I'm better off going for the OEM bearings.

Except for... the RHS balancer bearing

This obsolete part is quite expensive if you can find it so I went looking for an alternative. On the German language parts fiche you sent me Mezo the part number is 93313-92513 (with 6305 handwritten next to it). On the XT600 50T parts catalogue taken from tenere.co.uk though the part number is 93306-30541.
This bearing (the 93306-30541) is also shown on the parts fiche on cmsnl and partzilla as being the RHS balancer bearing for the 1984 XT600L.

CMSNL says it is superseded by 93306-30534 which is in turn superseded by 93306-30560. And I think the 93306-30560 here means it's a 6305 which would match what was handwritten in the 34L parts catalogue. Also the photo on cmsnl of that bearing shows "KOYO", "6305R" and "SH" (which I think stands for Special Heat Treatment) stamped on the bearing.

So, I think 93306-30560 is a suitable replacement for the 93313-92513 from the original 34L parts catalogue and at 22 dollars on partzilla it even seems cheap compared to the 160 EURO that the original bearing would have been. It seems right to me anyway but let me know if you think I got it wrong (or right even!). I'm still looking into it so will post back if I can find any more information on it.