. . . My bike has this with the drain facing towards the front of the bike. I've read that the magnet can hold the ball in the suction pipe closed. Is this at all possible?
Would this cause my problem ?
It's been known, but your 100ml down below after a run would suggest that's not the most likely cause I think.
Quite easy to reposition the sump plate to be sure though, and you should be able to check if the ball is operating. The pipe should have a ball retaining wire/clip a bit of the way up to stop the ball being sucked up to the top - if that is missing in action, problems can follow (been there on that one). I believe the pipe can also develop an air leak at the top where it goes into the case which would also affect scavenging. But I come back to your very reasonable 100ml . . . ..
All a bit of a poser.
You report clean, or at least OK plugs, and you don't report a huge oil consumption although I imagine it must be on the heavy side?
What is the oil return like in terms of consistency and volume of flow at varying rpm?
Is oil pooling in the head I wonder due to a drainage problem (although can't quite imagine how that could come about on one of these)?
If the guides and valves are good, and if the rocker oil feed is in order, then it does sound as if it might be a pistons / ring / bores thing. You say there isn't much wiggle on the pistons, but could there be broken or stuck ring(s), or even the not-so-rare problem of the 'wrong' rings having been fitted by the PO, allowing some up-and-down in the pistons' grooves, with a consequent pumping effect? (I have that oily T shirt too.)
Has to be something affecting both sides if the pipes are both mucky, so could 'just' be glazing I suppose. But in whatever case, it's probably going to need the top off again methinks unfortunately.
