Bike Slowly Dies after 20 Minutes of Use

Have you tested the coil? You can test them with a DMM, the factory service manual should list the ohm specs.

Secondary resistance does not matter. It is only given to see if the secondary changed, indicating a failure.

All you need to know is the primary is 4 ohms, and the coil is a standard size coil for a classic motorcycle, and not a "mini-coil".
It can even be a bit higher, like 5 ohms, but 4 would probably be better.

There are usually quite a few options on ebay, some even look exactly like the stock coil.

www.z1enterprises.com/dyna-dual-output-5...m-coils-1-dc8-1.html

Thanks for the link - I found one or two 4ohm coils on eBay; but since they didn't have the 14.5k resistance at the plugs I didn't want to buy them.

After loudhvx clarified the secondary resistance doesn't matter, I will likely buy the cheaper 4ohm coil which better matches the bike's specs; unless you recommend otherwise.

The Dyna 5-ohm coils are known to work well with the points-driven Kz's. 4 ohm is a better match, but I think the 5 ohm coils should work just fine.

My bike came with 2.3 ohm coils, and they are the best match for the ignition, but I actually use 3 ohm coils and they work fine. I can feel no difference from using the stock coils.

That said, the Dynas are kinda pricey. I also have a set of no-name ebay coils that are almost identical to the stock coils and they worked great at about half the price.