Join the Rough Stuff Fellowship. There are several members who use commuter bikes. I've seen some bloke in his seventies riding a Moulton up in the Cheviots and there was a photo in the journal a year back of a guy on top of Nevis with a Raleigh Twenty. They are nuts.
Can we have some sort of handicap - time bonus for the less you paid for your ride. I'll happily give it a bash on some £5 charity shop ladies shopper!*
Is there not a pretty decent length track coming down off the pentlands past castlelaw ranges?
Might not be as long as my childhood remembers it but I think it was a fair length. Also remember it hosting a race many moons ago!
If you can get discs and suspension that is period correct and you can prove it then sure go for it. I thought of sticking Honda Novio moped front forks on as this 70's bike had a 1" steerer fork.
I do have a twenty and yes I'm thinking this could be the right machine for the job. Also have a CWS shopper with 16" wheels though I reckon 18" could be fitted if anyone wants a project
An old 26 x 1&3/8th inch wheeled roadster would also do as knobbly bicycle speedway tyres in this size are available as are new wheels in this size. In fact a bicycle speedway bike could be used. A CX bike is fine as well as CX predates mtb. Or a tourer would work, they have slacker angles than a road bike and are reasonably tough.
Might be cool to tart up your chosen steed as well, cant have Dr S's Clunker being the only work of art