Newbie with T500 engine issues

[andyblue22]

Fresh to this forum so hi to all!

I'm based in West Sussex UK, near sunny Worthing. I've been biking for 40 years give or take, mostly 4 strokes but I did have a Jawa 350 twin many years ago! Hankering after a 2 stroke I've just acquired a UK 1975 T500M which is in good cosmetic condition but has a few engine issues.

It starts and runs but there is a considerable top end (I think) rattle under load. The left pot is running better than the right one, which takes longer to get warm, it will idle just on the left pot but not the right one (by taking a plug lead off and fitting to a spare plug to earth the spark). Starting to check things over yesterday I noticed air gaps in the left side oil lines, but not the right. These air gaps are appearing right back at the union where the single left line connects to the oil pump. It seems to be pumping oil, as the air gaps are moving along the lines with the oil, but even pulling the pump lever to max position the air gaps keep coming. The union on the pump for the left line wasn't very tight, so I tweaked that but it made no difference. I'm assuming oil is flowing in the right side oil lines but can't actually tell as no air bubbles to see move! There is oil in the oil tank and supply tube to the pump by the way! I want to run the bike a bit to get a handle on what exactly might need fixing with the engine, but don't want to run it in anger until I sort this oil supply problem.

So questions:
Suggestions for where the air leak can be coming from and how to fix it? (I think the oil pump only has one piston, so assume the left and right oil outputs are from a common chamber, but if so, why only air gaps in the left line?).

Secondly, any recommendations for bike engineers to do an engine rebuild, if it comes to it (rebore/crank rebuild)? I had a well known one in mind but was put off by several recent negative reviews and seems it may have changed ownership recently. Sure I can look online but that's a bit of a gamble so any personal recommendations would be much appreciated.

Thanks!