Thank you for all the replies.
With regards to efficiency, it sounds like the engine is not going to be enough worse than a younger one to be worth worrying about, but I am glad I had the opportunity to brace myself for the running costs! It sounds like it is going to use at least 1 gph at tickover rising to perhaps 6 - 7 gph WOT. I think I am going to need more than one 25lt tank! The Glastron hull is less steeply Vd than most ribs so there should be an efficiency gain there, but it will ride more harshly in a chop. One of the jobs I have for the boat is teaching scouts, which will be lots of low speed work. I'll just have to charge extra for those sessions
A1an, it is a shame to hear that your engine broke. I found that thread earlier and was impressed at you dedication. I hope that the splines failing is not a common fault! On the other hand I may well want spare parts when I try and get the thing to bits.
It is good to hear from Steve that the engine is slightly less ancient than I thought, also confirmed by matching with the decals - thanks T15.
I'm also glad to hear that changing the impeller is generally easy, thanks 9D280, and that normal people would consider using an engine this old.
The biggest problem left seems to be transporting the boat. It is tempting to just launch it and go, but I am definitely not going to try that. There is scary water on the way (Bardsey sound, Caernarfon Bar etc), I haven't done the impeller and I really don't have the experience. It would also cost a bob or two in petrol! Asking to borrow a trailer is a bit much as it would have to be adjusted to fit the boat. Buying a trailer is going to make this too expensive...
Thanks again everyone