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Maybe try one of these cheap plug testers. At least you could visibly see spark getting to the plug and rule out ignition. What do the plugs look like? https://www.harborfreight.com/90-in-line-spark-checker-63593.html?hftm_sc=532&hftm_source=google&utm_source=google&utm_medium=cpc&utm_campaign=21901271210&campaignid=21901271210&utm_content=171677806542&adsetid=171677806542&product=63593&gad_source=1&gad_campaignid=21901271210&gbraid=0AAAAADAHb4cCWCcSJ4qCMytUVmlv7v7bX&gclid=CjwKCAiAlMHIBhAcEiwAZhZBUlzpLzH9TR7YSC9i3f7bzmYEm7gw-Ai3uRRfne-Qjhh-zYy0O4G-khoCpwwQAvD_BwE

Disconnect at the manifold/header exit and run your borescope up that way. That distributor looks like just a magnetic pickup distributor, that uses an external ignition module. But your exhaust blockage sounds reasonable. Good luck. Post pictures.

Your compression test would seem to rule out anything mechanical in cylinders one and two. And yes, front carb supplies all three front cylinders, so I would not suspect a "no fuel" issue. How confident are you that the carbs idle settings for the carbs are where they should be - so both carbs flow the same amount at idle? And, when revving? Spark plugs can be dead right out of the box. I'd move plugs around and move plug leads around with them. I'd try swapping known good, perhaps from six to one and from four to two and vice versa. I think a Z engine will have a very hard time running if it has two totally dead cylinders. They may be intermittently firing a bit. So, new plug wires, new plugs, and new distributor cap?