I think I would look first for an original Mille miglia crankshaft and confirm the condition. The crankshaft will have stamping such as month and year for the date and an identification number.. I think one of these in usable condition is the most important first step

An engine block that has been machined to accommodate that crankshaft, and in other ways Would be next and it should be based on the first series, Fiat 600 motor that was originally 633 cc but enlarged to accommodate the Mille miglia crank shift and larger cylinder bores

I think for the double bubbles it was only the later cars where you could match a cylinder block identification number with an original Abarth ID plate on the car.

I think most of the double bubbles did not have an original Abarth ID plate but again there are some that evidently do that, I believe to be later ones.

Also, I would look for attributes such as Weber 32 IMPE Carburetor and larger valves and the cylinder head. Should like the block be of the earlier type based on the Fiat 633 series one 600 motors.

The ideal would be if you could match the motor number to a chassis by contacting John DeBoer

As above you want to ensure that the motor is based on the original Fiat 600 633 cc series one motor. The 600 D motors are quite different in very important ways, particularly the lubrication system. I think the two easiest ways to identify this is 1) the cylindrical extrusion from the cylinder block under the headers behind the fuel pump, which you will find on the early 600 block and also of course a regular crankshaft pulley as opposed to the centrifugal oil filter pulley on the 600 D motors.

If you can get an original 600 manual or copy of one which are found on eBay and Amazon, you’ll see the great majority of it is relevant to the series 1 600s but there’s a section at the end relevant to the 600 D motors

I hope this helps!