Probably nothing more serious than a restricted fuel supply, a thorough clean should sort it.
Start at the tank , clean inside, check that the breather hole in the cap is clear and that any gauze filter on the tap is clear. Next check the fuel pipe for obstructions and internal collapse, also check any gauze in the banjo where it joins the carb. If that’s ok add some fuel and hold the carb end over a jar and turn the tap on. If there’s and even flow for at least 30 secs that is a pass!
Then strip and thoroughly clean the carb.
If that doesn’t sort it the next step is to check for possible air leaks from perished gaskets or warped flanges weakening the mixture before it reaches the cylinder.
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