You need to find out where the crankcase is actually breathing.It sounds like it has been changed.
For the breather on the chaincase to be working,the drive-side mainshaft seal would not be fitted.This is normal for '70 onward bikes;they use no seal and have 3 drain holes in the crankcase to drain any excess oil from the primary back into the crankcase.
Your original breather may or may not still work.If you still have the drilled camshaft and the disc valve and spring are fitted,you could go back to using it.You would need to test if the disc valve is in there and working.
If you use the early breather,you could either
1./ Leave the bearing seal still out and the drain holes still drilled,and just use engine oil in the chaincase.
2./ Fit the seal,plug the drain holes (if any),and use ATF in the chaincase instead of engine oil.