Just in for all of us MkII owners!
The distributor cap and rotor on the ARD MkII for the 750 are off of an early Fiat 600 (early to mid sixties).
Note this is not the same cap as a Fiat 600D (later model), and seems somewhat hard to find .I got mine off of ebay, but could only find one place online to get the cap, and it was in Germany for $50. The rotor is the same as the ones for the later Fiat 1100's, but the 1100 cap doesn't have the 90 degree angle on it. The cap is probably the right size, but looks like a standard auto dist. cap. They're easy to find, and I don't think they would interfere with the engine when mounted, so it might be a solution also.
I bought 2 of the earlier caps though (and not for 50 bucks either
), so hopefully I should be ok for awhile...