Although yours is a very late model with the breather exit moved from the crankcase to the top of the oil tank, it's still possible to revert to the older-style breather with the "duck bill" rubber valve pointing over the rear chain just behind the gearbox sprocket. Remove the breather system completely, you can salvage the bits you need from it.
From the breather tubing you need a section with a short right-angled bend at the end, clip the short end onto the oil tank breather union, and cut the pipe so that the other end juts out over the rear chain. There should be a short duck bill valve inside the breather can, take this out and fit it to the end of the pipe you have just cut, using a short length of (eg) 12mm or 1/2" copper tube.
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Just as long as you don't over-fill the oil tank or ride it too enthusiastically (you did say it was the trials version) that will be fine, but excess in either department may lead to a bit too much oil sprayed over the back of the bike. If that is the case you can replace the duckbill with a non-return valve and fit a longer piece of hose to finish at the the back of the rear chain guard, which I had to do with my Electra-X.
The pictures show an Electra-X, which I thinks has a few more bits of piping involved in the factory breather system. If you have a tube leading from the breather can to the back of the timing chest, or one leading to the primary chain case, these need to be blocked off.
Hope this helps
A.