Miatas are wonderful cars in and of themselves. I've owned and driven four of them for 22 years and a couple of hundred thousand miles. Putting a stock non-turbo Miata engine in an early TR would add a lot of complexity for a not all that great power increase over a well-built TR engine, and you risk losing some of the wonderful mid-range torque of the TR motor. The result might wind up something like a late-50s/early 60s ALFA Spider only with less capable handling.
Instead of a conversion, I'd recommend that you simply consider buying a good used Miata and enjoy driving the bejeezus out of it. They're inexpensive to buy, have OK seats, heating and air conditioning, good-enough stereos, and a top that keeps the water out. The only downside is that you may find yourself driving it more often that the TR.