I am having a hunch about the crank seal left side, is there any trace of fuel behind the points cover?
Also check for splits in the rubber inlet manifold, you can spray WD40 on it when running to see if it affects anything.
Hi had a quick google, here are shops that has a 125DX in stock for breaking
http://www.browniesmotorcycles.co.uk/PHP/Breaking.php
http://www.motorcyclesandparts.com/usedspares_results.asp?brand_id=33&action=showspares