Another idea... get a round slide mikuni from a newer model 340 jag, change the throttle cable and your done. You can tweak the carb and change jetting if needed, and parts are available. You would need a VM 32 carb, 300 main and 25 pilot, and a flange adapter to fit the 32mm carb. A universal mikuni throttle cable.
carb p/n VM32-S $104
throttle cable p/n 932 $12
mounting flange $10-$17 depending on brand and mounting holes
choke cable p/n 938 $12
all prices retail and part # from parts unlimited