If you want a simple, safe, comfortable, modern and reliable car,
a 140 is the way to do it. All of the older weaknesses in these cars
can be overcome with not a huge amount of money -- new solid state
ignition, modern cam, steel timing gear and computerized fuel
injection will make these cars as reliable as any modern car. There is
no real need to put a newer motor in. Heck -- they're really reliable
even without all that stuff, they just need more fiddling.

If you want a super killer fast sleeper, you can put a V8 into a
240 without much trouble. You can also put a hotted up B20 into a
140 without much trouble. The costs in either case probably wouldn't
be more than a new miata and it'll be plenty fast.

If you want to make a cool, I'm the only one on the planet with it
kinda car, by all means put a B23 into a 144. It has been done before.
Of course, the B23 is really very very similar to a B20. They share
cranks, connecting rods, bearings, oil pumps, and so on.
chris