You've asked about K&N filters - I'm not sufficiently technical to give you a substantiated answer, but just by applying some shade tree mechanic reasoning... Look at the intake surface area of the stock S85 filter(s) and make sure your cone filter has at least the same surface area if not larger. Anything applying this criteria which fits on the intake pipe diameter should be good.
If you need more professional advice, you could call some of the U.S. shops supporting the BMW tuning industry, I bet they would be willing to help. Or, you could call K&N or AFE directly and ask for suggestions for filters fitting the S85 (leave out the part about the crazy engine swap
).
Good luck, can't wait to see it running around terrorizing the Autobahns.