Fiat + Miata = Fiata

[kmead]

So going back to the Miata track width of the rear suspension, a bunch of people have successfully chopped the Miata rear suspension subframe up to narrow it. When they have done this they have had to make new axles which is doable as well.

The front suspension still isn't really solvable due to the difference in the structures, but at least the rear suspension could be grafted in along with the more modern power plant, transmission and final drive.

[Sparky]

I found a parts car with a working engine and good transmission so I shelved this idea for now.
I did pick up a lot of relatively useful info. The 6spd transmissions (with the external webbing) are wider than the 5spds, which are both wider and longer than our stock tunnels; I believe they're about 30.5" long, so you'd have to extend the raised portion of the tunnel as well as cut/weld/beat it wider...

The 1.8L engines are 1" longer than the 1.6L engines. You can use Mazda Protege or Ford Escort GT 1.8L oil pans on the Miata 1.8L BP engine to make it a front-sump. I believe the same can be done with the 1.6L engine, however I had some trouble verifying if any of the fwd cars with their front-sump style pans were really compatible (the pan is normally cast aluminum and 'structural'...
Ultimately the engine is going to be longer/closer to the radiator, an extra inch more so with the 1.8L. I got a lot of different numbers and didn't want to buy an engine/trans without being able to measure things out in person.

If you are looking to go this route, the '99-'00 1.6 or 1.8L engines are by far your best option. They make the most power with the simplest controls and have the cam sensors on the front of the engine rather than protruding from the back of the cam which would require cutting into the wiper box/cowl/firewall area. The JDM 1.8L BP5A like the US BP4W has solid lifters, but it also features a bit more aggressive intake came. For best performance, you want to swap out the intake manifold for what's referred to as a square-top, it will also simplify the efi if you're going stand-alone.

[DieselSpider]

Haven't looked too closely at this but typically the issue with swapping the front suspension will be with the height of the upper mounts of the McPherson Struts which may require some creative mods to the fender and hood along with everything below them.

The longer 4's are a problem. The Isuzu 4F1B Diesel in mine leaves the engine almost touching the fire wall, water pump shaft almost kissing the radiator and very tight clearances to the center link using the stock front suspension. The rear shift re-locator for the MSG5 transmission appears to have put the shift lever too far back so they used the truck style shift lever which is so far forward that it almost hits the center console and then they had to do some artistic bending of the long lever to put the shift knob into hands reach making it sorta work. That makes for a more vertical splayed out shift pattern that takes a bit of getting used to.

Can be a long road of compromises and such.