It has been a while... but I believe there is a round access cover about 2" in diameter on the right hand side of the bike, forward part of the engine.
That corresponds with the end of the exhaust camshaft.
In this photo, it is chrome plated. They are held on by two philips head machine screws.
The points themselves are mounted on two plates. The first plate adjusts both sets of points, the second adjusts the difference in the two sparks.
That said, you might be on a wild goose chase. Most of the older T120s and T140s have already been converted to Boyer electronic ignition systems. MUCH more reliable, and much less maintenance. Smoother, and more precise as well.
Boyer conversion or not, the access plate in in the same place. It just might be a "pointless" ignition.
See these guys - they have very good telephone support - and will have Boyer kits in stock.
British Cycle Supply, item
331 Series Boyer Electronic Ignitions