Mosin On the bright side it runs OK on one cylinder....some bikes this week don't even cough.
Oilflow is easiest check. Try swapping the banjo bolts in case there is some hidden difference. Next, with the box off, check the rocker shaft is clear, warm the box to easily remove the shaft.
If all in order, you need to have a look at the valve. Check for bending and/or bow in the stem, and witness marks of higher loading along the length. Piston crown....has it touched? Maybe worth swapping the inlet valves over, it may just be a case of on the limit tolerances for guide and stem. If you can not find anything simple amiss then bite the bullet and poke a bit more from the guide, like musky says, it will only need a light internal polish, honing will be fine.
Swarfy.