Alex,
The easiest thing to do is to fit a degree wheel to the end of the timing side of the crankshaft while setting up. However it sounds as if the engine is further along than that. You could stick the degree wheel to the face of the flywheel with double sided tape, or subdivide the circumference of the flywheel into degrees.
The era of your machine, I would expect it to have the "IN.OP.20" camshaft fitted. If so it is timed for the inlet to open 20 degrees BTDC. It is possible that you have a "10.50.63.20" cam fitted. That is timed for inlet opens 10 degrees BTDC. According to factory literature the valve lash on the later cam was 0.002" inlet and exhaust when hot. This setting always worried me as I would expect quite a bit of expansion in the cylinder and head, and I would be worried the valve clearance would be nil or slightly open when cold and first warming up. So I always set my '34 OHV Douglas for just being able to rotate the pushrods freely (about 0.002 inch) when cold and then if anything it would gain a little more clearance when hot. I do not recall seeing factory valve clearance figures for the "IN.OP.20" cam, but would expect the factory settings to be the same as the other cam mentioned. I have the "IN.OP.20" cam fitted to my '34.
You have the eight splines on the camshaft, which are not perfectly aligned with any one tooth. So playing about with that can alter the timing by a fraction of a whole tooth. Also the crankshaft pinion has three keyways to intentionally allow one third a tooth division for timing purposes. With that you should have no trouble getting it close. But it is not super critical, clearly from the above Douglas experimented with cams giving from ten to twenty degrees for the opening of the inlet valve. That alone will tell you that it is not the camshaft being out a degree or two that will prevent the engine from running.
The factory literature stated that there was no need to alter the cam timing regardless if one was running petrol-benzol or straight petrol, and indeed both cams were used in Dirt Track machines and road going models.
The factory literature also gives the ignition timing as 45 degrees full advance on DT motors. This is with the anti-fouling pockets still intact. If they have been drilled through and long reach plugs fitted, then the ignition timing is probably going to require a more conventional 35 degrees full advance to compensate for the faster ignition of the main charge. On the same '34 Dougie mentioned above, I ran with 42 degrees advance on straight petrol. Do not overlook the fact with the magneto that not only do you need to time the points to break at the optimum moment (moment of maximum flux), but that the brass segment of the slip ring is adjacent to the plug lead that you want to fire. In other words do not have the embarrassment of timing for the front cylinder only to have the spark occur in the rear cylinder! As far as the internal timing of the magneto (maximum flux), I think that as long as you are using original, unmodified point plates, that you really only have to worry about fitting CW or CCW assembles. The original key and keyway locations take care of the internal timing for you. However, and someone else can jump in here and comment, I believe there are CW and CCW slip rings? If the wrong one is fitted (during a rebuild) the brass contact may be too far away from the carbon brush to fire the plug.
-Doug
