Honda XR400 trailbike project maybe????

My mate has owned his Honda XR400R since roughly 1998. Last 6 or so yrs its been dormant in the garage and now in his new house, the shed.
6 yrs ago it was getting its usual 'used and abused' offroad when a mate riding hit a half-submerged pipe at 40mph. Cartwheeling the bike over and over and breaking his leg into bits.
Leg fixed but bike wasn't.
Rear subframe twisted a couple of inches/broke some welds. No problem as i do welding at work.
Rear shock has blown due to the huge impact.
And lastly, the bike now jumps out of gear under load unless you hold the gear lever up with force. Its ok if cruising but if going round a roundabout leaning over and you throttle to exit, it'll ping out of gear and try to have you on the floor!
Bike is tidy apart from this, and lack of use not doing it any favours(carbs/stale fuel etc).
Oh and its a bit loud as the baffle got blown out!
What do you think? Should i offer him some money to take it off his hands for a project?
Worst case is a replacement engine, rear shock and weld subframe again. Maybe £300 to fix?
They go for £1100-1600 on eBay.
Will defo have to fit a quiet exhaust, and from what i remember its a pig to start when hot. Kickstart only.
What say u???
Its about as mad and anti-social as a bike can get, but thats why he loved it sooo much, and the over-run backfiring was awesome!