Current limiting a starter motor
This should be a comment, but I can't create comments yet. A starter motor can afford to run at 100 A because it does so for a very brief time. I don't think limiting it to 40 A is going to allow you to run it continuously. As an estimation, what you've done is decrease heating power by 4 roughly and this allows you to run it 4 times longer. It is not as simple as that but it is a ballpark.

What you should do is look for an engine with a nominal current at least as high as the one you need.

Other than that, a resistor is a very inefficient way to limit current and only affordable for a brief time (i.e. starting).