Dont use the To-hit modifiers!
One component will share its ability to all of the troops on the planet.
Instead, I suggest simply merging your engine with your cockpit.
That way, it is still required, and you can pay for your extra space with a simple cost increase.
Alternatively, you could go with a GritEcon style troop system...
It has extra troop sizes, but the main thing is that the hull build cost is proportional to size cubed. The light infantry are very weak, but you can build them 200 per turn.
The heavy tanks cost tons to build, but can carry thicker armor and bigger, more efficient guns.
The result of that, is you need to have some heavy tanks to dish out damage, and you need infantry to act as cheap-to-replace "ablative meat" armor
