This last weekend I managed to finish up fitting the seat covers. I had ordered these covers from an Italian company called Volcano Industry based in Sicily (probably named after Mt Etna!) who I originally found on ebay.
The kit turned up fairly promptly and had a run of elastic sewn on the edge which you can use to staple it to the seat base. The kit is very tailored and fits very snuggly, so a little effort is needed to pull it around and get it in the right place, an extra set of hands would be useful!
The end result is as good as the original if not better, the cover follows the curve of the foam and looks factory. Volcano also can offer alternative colours for the back pad for Sintesi use.
Bottom line: I am super impressed, Volcano have over delivered in quality, fit and finish and I can't recommend them highly enough. If you are restoring a Sintesi, buy this kit.
Full disclosure: I paid full price for this kit, I just believe in praising and recommending a great product when it warrants it.
https://www.volcanoindustry.com/shop/seat-cover-aprilia-af-1-sintesi-1989-1992/