I think you are right in saying that it is fuel starvation, and that's where I would start looking. Pull the fuel line off the tank, check you have a good flow out of the petcock, and then take it from there. Sounds like you will need to remove the carb and dismantle - a fiddly job, but not difficult. I'm guessing there will be a blockage in there somewhere. Main jet in the RHS carb would be the obvious candidate.
If the carb checks out OK, I would run a full check on valve clearances, compression etc before going any further. Also check the carbs are synced correctly. A search on this site will bring up the details.
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