I have wrenched on a few bikes...the reason for that constantly leaking head cover
could be excess heat.I had a few customers who made the engines HOT sitting in traffic and revving them alot.I remember a guy that I sold an 83' GS1100E to(that bike had an oil temp. gauge) and I told him to rejet the carbs after he installed a Yosh.
4 into 1 pipe, but he didn't.The temp gauge went over 310 degrees & that was too
hot;I checked the cycle and it was no worse for wear generally, but all that heat had warped the head(valve)cover & try as I might,I just couldn't get it to seal.
The actual Cylinder head & other components was fine,just the "cover" warped.
I think you can pick up a head cover for small $ on ebay,if that's the problem.
I would check(smell)the condition of the oil before I buy it;make sure that it is not a
"sludger".