Martin, looks like engine mounting point, not an issue. Mount up the motor and run it through its paces and then check for leaks. On the rear wheel- go with the 94 or whatever you can find to get a disk mounted. Adding the 30+ pounds to the weight for auxiliary fuel tanks, the stock drum will wear out real fast at race speeds. I raced a 91 factory prepped HRC 626 for a few years and went through rear brakes often, sometimes on only a 250 mile race. Have you looked into the removable sub frame option yet? Copy a CR 250/450 design. Find diagram on the web. Add sleeves at mount points and gussets with holes drilled at all weld points. I will look for pics of my old girl to show you.
Coil- double it up with two units.
Ignition- update to XRL 650
Clutch- heavier springs and plates.
Oil cooler- use XR 400 unit, mount 2 up one on top of the other- cool oil,is your best friend.
Frame- gusset the rear mounts where connected at swing arm- just peace of mind- poor welds here.
Wheels- heavy duty spokes, especially on much heavier rear. Stockers will snap on a square edge high speed bump.
More if my feeble mind can recall what my team did back in 91-2.
Keep the posts coming. - Jerry
KTM-2008 EXC450R, 2009 690 E BMW- 2012 GTL 1600-sweet, 2011 R1200GS Adventure Honda- 2000 XR650R- 2006 XR650L- Yamaha- 1972 XS750 Tracker, 1971 RC5- BSA- 1945 M20, The newest member to my stable. The list goes on but why bore you... 16 and counting. Its an attachment issue.