I know this a an old thread - but I wanted to comment on this for clarification. We put a Miata engine into our 24 Hours of Lemons XR2. Miata engines are easier to find and cheaper than Mercury Capri XR2 engines (even though they are the same thing!)
A 1.6 Miata engine will work in a Capri. You'll need the Capri oil pump (or modify the Miata pump for the dipstick tube if it's a different crank and you can't use the capri pump -- you'll also need the pickup tube from the capri pump for this). Everything else bolts off / on. There are threaded plugs where water / oil inlets and outlets differ for the different cars, but in the blocks I've seen, everything is already drilled and threaded for all applications.
Miata heads will work also - it's a matter of changing which water outlet is plugged - just remove the old freeze plug from one and insert a new one in the other. Obviously the turbo exhaust(?) cam has different timings, but the distributor fits into the same space as the cam sensor on the miatas. Everything is bolt off / on.