A rough old barn find, but it’s in near stock condition – even the numberplate looks original. The seller has another MBX for sale, a black A-reg model, so it would be worth trying to do a deal for the pair and building one good bike from the two. A new fuel tank will almost certainly be required – the owner says he’s got a spare he’ll throw in. The MBX50 was painfully slow and tuning parts are scarce. Better to hang out for an 80.
That looks like it’s been stuck in a shed for a while [last taxed in ’91 – JM]. The thing with these little bikes is that they can often cost as much to restore as something bigger and faster, so you’ve really got to want it.