Thanks Legion and Sam for your comments. Glad to hear the Hyde half-race cams are good as they are drop-in with the standard timing marks and followers. It's a 1982 model so I think it probably has the later main bearing set-up anyway, but yes definitely the thing to do. Anyway discovered the cause of the problem: the cage on the starter sprag clutch has broken up and a small bit of the cage got caught behind a pinion (the whirring noise) and I reckon a bit of aluminium may have caught between the teeth as well (the clunk when it disintegrated). Apart from the small bits of debris and the obvious damage to the sprag itself with just the faintest of witness marks on the shaft that runs inside the sprag, everything else looks okay. It may have been that way for some time as the e-starter has been making graunchy noises for the past year or so, but still worked (a bit half-heartedly admittedly), and I even have a new sprag clutch waiting to be fitted. I'm tempted to possibly just flush the crankcases as the only way any shards can get into the oil circuit would be through the communicating holes into the main crankcase and via the oil return line. They will have been caught by the oil reservoir filter. I'll check the oil pump tomorrow to see if there are any witness marks in the return side bore. Anybody care to comment on the above? Anybody have an experience with self-destructing sprags? Thanks!