If one cylinder is sparking at the right time and the other isn't, I'd suspect some combination of the wrong rotor, cap, and magneto. The Cat pony motors either fire 180 apart or 90. I haven't been inside a D6 U-series pony but if it has the same crank layout as the D2/D4, then it'll be a 180

Langdon, see how that brass tube sticks out of the cast iron "bullseye". A problem that sometimes occurs is that the gasket loses the inner part / fails to seal correctly, and that can cause running problems. Also, if you flip the fuel bowl over 180 from that shown in your photo above, there is a small lead plug on that passage that runs across the bottom of the bowl. Check that you can run fuel through that passage and if it seems that it doesn't flow well, you can drill the lead plug out carefully, then using a small drill bit (I think 1/8") in your fingers, "drill" that passage out until you get to clean metal. Use a .177 cal BB pellet with some epoxy to reseal the opening. If that all works and you get it running but it won't idle, there's a passage on the side of the throttle body that runs to the idle "jet" which is just a small brass plug with a notch in the side. That also needs to be able to flow for the pony to idle well