Earlier this year I inherited my Dad's 1960 500 M18 that had been sitting for 3 years. He'd been riding it for 40 years and had really neglected the bike. Difficult to start, performance was flat, it smoked, the plug kept fouling, compression wasn't good, brakes were terrible, it wouldn't charge etc etc etc.
4 months later:
Rebuilt top end: rebore, new +20 piston, new valves, recut valve seats, new valve guides.
Thorspark electronic ignition.
New brakes, front drum skimmed as was 6 thou elliptical so brakes juddered badly.
New alternator and solid state rectifier. New Motorbatt high power gel battery.
Now done about 300 miles and the difference is enormous. Starts first kick hot or cold. So much more grunt even though I'm still taking it a bit easy. Lights work great. Brakes good.