I'd say at a guess the 12S would fetch £200, the 160 £250-300, in good order. These prices are serviced with same-spec bits, maybe a tad more.
Modding it with exchange boards seems like a great scheme as it's reversible. Best of all worlds, like installing underfloor heating in your Jacobean farmhouse but retaining the original fireplaces and stonework.
I'm wary of getting too precious about these things. If it really is the Jaguar D type that won Le Mans then you'd be a fool to start "tuning" it. Better to buy a fibreglass replica with a 4.2 engine and play with that. However if it's just some 80s Porsche, then sympathetic mods go a long way. Better that than preserving it as some smoky, rattly 30 year old banger or paying a fortune to have something "restored" and ending up with all the performance of a modern Astra diesel. It's for you to decide where your amp lies on this continuum - I'd say it's an 80s Golf GTi, no great history but not as disposable as a Fiesta.