Good luck with the sale of your jacket. I advertised an Irvin jacket (size 42) in excellent condition for a similar price to yours here on Blatchat in January and was surprised that I got very little interest.
Irvin Jacket (Aviation Leathercraft). Size 42 with front pockets. The jacket is in excellent condition having been worn very little. I can Email pictures to anyone interested. £450 plus P&P.
Good luck with the restoration (perhaps more of a reincarnation judging from the pics posted!!) I am no expert on S4s but seem to remember (as you are probably aware) that the front suspension was a mixture of Europa and Triumph parts. (My brother rebuilt an S4 many years ago). Perhaps it maybe worth enquiring with the relevant Clubs if any parts are potentially available.
Will a full hood fit over a "standard" FIA roll bar (single diagonal as supplied by Caterham) and, if so, is the hood frame needed or does the roll bar adequately support the hood without such issues as inadequate "tension" of the hood or undue gaps to the side screens?
There is a Caterham bonnet for sale on EBay, stated as from a1998 1600GT. It has the bulge on the bonnet to accommodate the single downdraft Weber, although the bulge is “solid” without the front section cut away to act as a “scoop”. You may be able to adapt this or use it as a basis to form your own. Ian