Steve,
They are entirely different engines, and the little Spit motor will not have the low end torque your TR needs, nor will it simply slip into the TR. It would be a more practical solution to look for a Toyota engine of about the right size and power. In our shop, we've done quite a number of "power swaps", and I will tell you that getting really satisfactory results depends on some sound engineering and tons of attention to detail if you want the end product to be enjoyable. But there is a nagging question: What's wrong with using a TR3 or TR4 engine? The car was made for it.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon