I would personally stake a lot on the new breather... you may find that that takes care of your "oil pressure" problem....
The other thing to check is that the oil filter is in its place correctly. There is an oil pressure relief valve (ball/spring check valve) at the back of the oil filter chamber. Anton has something to say about that on his website
. ....see bottom of page for oil pressure relief system.
Good luck, nice bike!
PS i would love to hear what those pipes sound like!