Your alternator and rotor need to be in good shape to use the cap. I used to run without a battery all the time, but when I put it back together in 2010, it would make spark, but no start. As soon as I hooked up the battery, it started. It will still run without the battery, but I'd probably need to get it on a good hill to build up the charge.
If it's still the original 2MC, it's most likely bad, so remove it or replace it, you can find lots of rather large capacitors like ludwig's cheap.
Dave
69S