Add to that, the constructor modules' cost is 80 and the size is 35. How many engines could you add within those costs and sizes?
The size of the Impulse drive is your limiting factor. You could probably fit 3 of them because of the SizeMod, while still keeping it within the size of the Constructor Module. Given a speed of 2, and the hull having at least a speed of 1, 3 Impulse drives would give you 7 speed at a much lower cost - 24 vs. 80.
If you go up in engine techs, the gap widens even further with both the cost and size.
It makes no sense to use a Constructor Module as an engine, no matter how many you can add. Adding more than 1 just makes things worse. And as it has already been pointed out, there can be only one. And that one is, no brainer, a bad choice to make.