Any question answered...

52 / 36 semi compact rings from praxis would get you close. I thought you could replace the spider on the SISL cranks, independently of the other components - might be the way to go.

Yep - just change the spider. Plenty of people out there who'd swap a 130 for a 110. Spiders from the original SI cranks will fit, too, so there should be plenty around. I picked up a complete set - 2 road and 2 MTB from ebay for less than £100.

Or SRM / P2M etc.

Or SpiderRing.