Yes, the magnatometer needs to sit horizontally level in flight attitude hence the design of the Rans mount. See the Dynon downloads and installation guides. I also use aluminium anchor nuts and stainless steel hardware

Yes, easily modified the Rans mount to take the Dynon unit

there are at least a couple of builders who have installed an IO-360. They have split the top cowl and installed a centre splice to widen it to suite. Thinking I saw their posts in the Ransclan forum and on the S-21 builders Facebook page too. Don't know that they have flown yet?

Dave, I’ve acquired an IO-320 to install in an upcoming project, my kit just shipped from Rans to Australia. If you checkout or post the question on the S-21 builders Facebook page and the Ransclan S-21 forum you might get a quick response. There’s quiet a number of O-320 /S-21 operators now. I’d be interested in performance comparisons from the variety of engine/ propeller combinations , chz Rob