IF I recall correctly, didn't Bert F. use a 495 with maybe one of the Sharps on board at an air field in early 80's for a speed attempt?
In any case I reckon 200 IS feasible, just need a strong 500 that will rev and the right gearing, the problem would be getting it to those speeds within the measured run area AND is the run not an average, i.e. has to be achieved both ways? I have had a 500 mx bike checked on radar at over 180kph, a long time ago and the engine was fairly stock, just correct gearing and plenty of room to get there, old style KR11 radar back in the 80's, was on tar though and street tyre fitted on rear, very scary with nobby front although it was a K139 (remember them boys?), also had a DR650 clocked at 160 ON DIRT about 10 years back, that took several kilometres but was also radar checked, I wouldn't have believed it until I saw the readout! AND no ticket involved either time.....just laughs all round and plenty more trys but no faster, have no idea of wind assist or if slight down hill, in western NSW.
Kt