Plug coil is easiest & cheapest to replace for a try but symptoms like that with no spark when hot point more to source coil. But in saying that, it's common in cdo source things because they're such fime windings. Not so much in points mag source as a much more robust coil. But--50 years old--possibly--!!. Condenser I'd think you'd still get a flash at the plug resting on the head but who knows. Get a DT400 late model cdi mag or a "79 250 coil mag.

Go for the easiest one first. Test the two circuits of the coil. The two tests are in the manual for your bike. Probably didn't have to tell you that .

Replaced old coil with Emgo coil 24-72400 + NGK LZFH non resistor plug cap and BR9ES resistor plug. Needed to unscrew the spark plug terminal to get it to fit the new cap, but my first test of the hot start issue is a success, woo hoo! Hope it's not a fluke. I noticed the old coil had a small amount of fresh shiny black goo oozing out in a few places, it stood out since the rest of the coil has the usual road dust. Many thanks for the advice from the awesome members on this amazing forum! 

Congratulations!
 

Sound positive & hope it continues.