HELP! My rear rim is cracked 3 weeks before I do LEJOG

Whilst truing the wheels on my tandem today I spotted that around almost half of the spoke holes on the rear wheel there were 1-2cm cracks radiating out and some of the spokes were pulling through.

Obviously im not happy to put 1000 miles on this wheel so urgently need a rear touring wheel, Its a tandem so needs to be strong, the one I have at the moment is 40 spoke but 36 will probably do.

Can be either freehub or screw on freewheel and 700c.

Please help I don't have long!!

OLN?
I have a NOS pair of Mavic T215s built onto unused older style screw on Pelissier hubs, 36 double butted spokes 3x. Rear is 126 IIRC. Hubs are superbly smooth, were built up by a VCC member and are absolutely bob on.
I've being fadging about over using them on my Carlton for quite a few years as it was cold set some years ago to take 130 but I've located some 27" rims that I'm having built up with HF hubs and a longer axle.
I know you were really just after a rear but thought I'd offer them up.
£123 including Parcelforce 48hr delivery so theoretically could get them to you for Wednesday, or collect from Letchworth Garden City minus the £13 postage.
(Pic shows with tyres but not included)
Alternately I've a 40 hole MA2 laced to a Sansin sealed bearing screw on hub, bags of wear left in the rim but the axle cracked for some reason, runs straight, spokes all good, it'd need a rebuild obviously. I can disassemble and send the rim on its own via Myhermes as it's cheap as chips and reliable for £28.

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If you can give me the over locknut distance I can look into sourcing you some parts and building you a wheel ASAP. PM me if you're interested.