Hi and welcome.
The overheating issue was probably headgasket failure, one of the main reasons for so many of these going to scrap as garage quotes wrote the cars off. It's not such a difficult job if you know the cars well and know the shortcuts. It's also cheaper and better to buy the metal headgasket from the newer model st205, but you need to get the head and block faces very flat & clean. If the engine is already out and you are planning on running increased power it's worth doing first.
A worthwhile source for parts for these is TCB in Cornwall, excellent service, good prices and a lot of knowledge to get you the right part.
www.tcbparts.co.uk If you have any questions ask away, or start a project thread then people can pass on information as you go.
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If at first you don't suck seed, try drier grain.