I have the kit; its next up on the building board. Found it on eBay, but paid a premium price. Apparently there are a lot of folks interested in this kit, since the bidding was hot and heavy. I won because I screwed up and added one too many zeros to my last bid... I saw another NIB kit there not long after, and it sold for even more than I paid.

Anyway, even though I paid far too much for it, I'm looking forward to building it. It is indeed a vintage Sig "builder's kit". However, the entire top deck is a plastic part. Not sure yet if I'm going to use that, or make a former/stringer top deck. One of the custom glass shops, Stan's or Fiberglass Specialties, I forget which, offers a duplicate of the kit cowl in fiberglass.

Regarding engines, the nose on this model is rather small. A 91 four-banger will fit, but needs to be inverted for best appearance. Otherwise, the jug and muffler is gonna stick way out there.
I'm putting a 61 FSR with Pitts muffler on mine.

So, I'd buy it. Even if you decide not to build it, you can unload it easily enough.