Idea number one: Give each model a DX,DY vector. Instead of moving you directly, using "Cruise Speed" or "Full Speed" would instead alter your velocity. The only problem is, I don't think this could be incorporated well into the movement code as it currently exists. Come to think of it, a DX,DY vector wouldn't work well with a tile system where there's only eight directions of movement either.

Idea number two: Cause each model to drift in the last direction moved unless they perform a full stop action. The main game loop would now have to make a separate check for drift in addition to the check for completed movement. The drift would happen at the same speed as the model's last move... I know, it's not realistic to have a maximum speed in space, but sacrifices must be made. One problem to avoid with this one is that it could result in the model moving two spaces at once if the drift and direction of movement oincide.​