It's a bit of an open ended question. Could do with a better understanding of what you wish to achieve.
Back to original spec? Same speeds and ratios with current availability? Modern spec with different speeds and ratios?
Then what does it have now and how much of that is serviceable and worth keeping?
There might be a brochure for this bike, I haven't looked in the archive, but Thorn have always built bikes up to order, so there's no guarantee that any of the recommended builds were used, though most buyers wouldn't have strayed that far. Plus, do you know if this bike is still as supplied? Unless you've had it from new it's quite likely some parts have been changed.
If the frame and wheels are good, it needn't be a huge task to get the bike back to being as good or better than when new. It does however need a clear plan to not fall into the rabbit hole of incompatibility.
