Barry,
Both welding up the taper and building it up with electroplate have been tried. From what I heard the weld leaves the surface too soft (the original surface is case hardened) and they are prone to fracture where the weld stops. This make sense as besides the transition in metallurgy, the case hard surface adjacent goes through a rapid heating and quenching (from the mass of the journal) leaving an abrupt transition from dead soft to dead hard. And that creates perfect place for a stress riser to initiate a fatigue crack.
A friend tried hard chrome plating to get around the deficiencies of welding. This lasted two years of intermittent sprinting before it snapped the taper off. Oddly enough the flywheel did not go belting off but stayed with the bike. The shaft broke clean at the top of the taper where the plating stopped. It seems again the likely culprit was a stress riser. Aggravating the situation was the low coefficient of friction of the chrome, which made it difficult to ever get the flywheel taper stuck dead tight. It seemed tight, but probably was able to shuffle slightly.
Depending on how badly torn up the surface is, you may be able to get it to clean up 80-90% by skimming the taper smaller. You will need to grind the taper or turn it with a CBN tipped tool. Some bad tapers and other crankshaft ills can be seen in this post:
https://www.douglasmotorcycles.net/index.php?topic=402.0Before going to the trouble, have the crank Magnaflux tested for cracks, particularly cracks from the corners of the Woodfruff key seat.
For the flywheel, I would be more inclined today not to sleeve it but build the inner diameter up with weld and re-cut to an undersized taper to compensate for that removed from the crankshaft. The flywheels were not heat treated (though the taper probably would have benefited from case hardening.) You could use hard-face or Stelite filler metal, but probably regular mild steel MIG welding would be good enough and would put the minimum amount of heat into the hub. I have not had to do this myself, because the job was always done in conjunction with machining a new flywheel; which was just made to suit.
Camshafts. Well which one do you have fitted? Douglas used several different timings, the IN.OP.20 and 10.55.63.20 are the most commonly seen. I have a drawing for the IN.OP.20 cam. These were used in the 1934-35 OHV road machines and give a nice tractable engine on a single carb. And of course they probably gave good torque out of the turns on the cinders. I have been told they tend to have too much low-mid range power for sprinting effectively (the bike wants to rear-up), and that better overall results (shorter times) can be had with timings that sacrifice low end for better high end breathing. But for a 3-speed road bike torque is probably more desirable.
Plenty of folk have welded up cams and had them re-profiled. Not sure what they used, and maybe neither did they! Cam life was probably not a major concern. I have heard of several folks in the UK using a material call (I think) Comalloy. Some sort of carbide micro-spheres dispersed in a bronze binder. It does not seem to be availbe in the USA. It is applied as a brazing operation. I can hardly believe it works, but I know several that swear by it. I would not mind finding out what would be best along the likes of a welding alloy, as I have some camshafts that could do with a weld and a regrind. You will also likely find the tappets are knackered as well. Presumably these should be welded up with a slight different alloy to avoid the engineering no-no of rubbing like materials together. If you have access to a medium to large surface grinder, there is a fixture you could build that would allow you to grind your own camshafts. The principle is like the rear swing arm of a motorcycle.
-Doug
