Send carbs away for recon. Before you do - check advance curve throughout rev range. Best thing to do is advance it until it pings the retard it a bit. Could try with a colour tune or check for air leaks (spray carb cleaner at spindles - may hear engine note change). I’ve had some luck with carb recon before, but also chased my tail for ages. Recommend you get a recon pair. Doubt you’ll look back.
Andrew Turner did my carbs (01476 860555), great job.
Low power could be caused by ignition timing. Not sure what your setup is, but expect it’s a Lucas unit. I might have box of these somewhere. They can be repaired by H&H near Birmingham.
Normally I advocate doing everything yourself, but having a professional rebuild some of these items with 50 years of ware can be very worthwhile.
last this - have you done a compression test. Worth a check before you go to unnecessary expense.