Yes, never fill to the top mark. Check oil when it's been running for a while and for sure you see oil pumping back into the oil tank by removing the oil cap. Everytime I start the bike I check that, it means you have oil to the top end among other things. You'll see oil squirting out of that tube that's visible in there. After a few minutes it should start to clear up and run sort of solid oil. You can check the oil right after a run, but don't let it sit and then check it. Keep it on the low mark unless it burns excessive oil. Keeping it full or too full only makes for oil problems.
Remove the primary, clean the grove and cover and just smear silicone on that round gasket or get a new one. Then smear a bit of silicone on the cover flat and put together. Don't overtighten or you will bend the cover. I think there's a torque value on that. Might want to check that the primary is not too full of oil, best way is replace it.
I have a cure for that hole in the oil tank, but it's rather complicated. You can seal that hole off, but then you need to 'breathe' the oil tank to atmosphere some other way or you'll start pumping oil out of all the other seals/engine.