Guess you mean S135 as this was the most common UK fitment. The S155 was only fitted to late 1300 cars here, like Lidos - there was a thread on this somewhere, but IIRC the 1300 advance curve is slightly different than the 1500 so I'd try and stick with another 1500 era S135 (if you can suffer the dizzy cap outlay) or move to the later era S178 as you suggest. The S178 from another X1/9 would be my vote as the caps are still decent - however, as these distributors don't use vacuum advance (just like the S135) they have a plastic blanking plate to lock the advance mechanism - these are very fragile and easily broken. The contact set/points for the S178 are more money than the S135 as they are a cassette type with easy hex screw adjustment, but more importantly it's supported on the shaft to counter any wear. You can get a Lumenition optical electronic conversion kit to convert the S178 if need be. I'd be aware of other distributors that are taller or that you can't get a right angle cap for, as these will unnecessarily foul the plug leads on the inspection panel (and you can't use stock x1/9 plug leads either).It is running a s138 dizzy
I just picked one up like that. I think I need to add a companion module to that as well, from a Yugo or some later Fiat.Alternative for you is the distributor from a fiat uno.
Something like this, its an electronic ignition version that fits.
https://www.ebay.co.uk/itm/132873845168
I don't recall which cap you need but you should be able to find a 90 cap for it and it more or less fits (as I recall it might need a little modification to the access panel)
ps I have no affiliation with this ebay auction, it is just an easy way to show what I mean.
I remember reading that thread and yes sounds like the same physical dimensions, possibly it was a aftermarket copy of the uno one.Some time back Jeff Stich posted about a aftermarket electronic distributor that will fit the SOHC, and is was very affordable. I believe it is slightly taller than the stock dizzy and therefore requires a little clearance to the access panel. You'll have to review the thread to find out the details:
EI Distributors - too good to be true?
I've seen a certain So. Cal. eBay seller listing various electronic ignition distributors for Fiat Uno/Panda/Tipo, looks to be offering both the block-mounted type & the cam-end-mounted type distributors...xwebforums.com