Well, I think you may have a few problems here .
First the uncontrollable engine rpm's, the usual culprits are unsynchronized carbs, or a carb synch done on a 'cold' engine .
Air leak between the carb and cylinder head .
Other possibility, is the centrifugal advance mechanism in the ignition 'bean can' is stuck in the advanced position .
Valve clearances that have closed up to nothing, is another possibility .
If you don't know this already, the '81 model year BMW bikes were the worst year for valve recession, the valve seat alloy ( a new alloy for this year) acted like an insulator and kept the valve from transferring heat the the valve seat/ cylinder head .
The valves on my '81 R65 needed to be replaced at 45,000 miles .
Your loss of power may be a carb diaphragm issue, they get brittle with age and crack/tear .
The 'choke'/ enrichment system, may be stuck in the choke or starting position .
