If you are using HePlar, then you will be limited by the exit velocity of the plasma from the exhaust nozzle.
Realistically, you would be limited to about HALF that speed since the difference in velocity would be reduced greatly reducing the thrust of the drive.
Current theory puts an upper limit at about 0.1 or 0.2c, but using gravitics and HePlar, maybe you could get to 0.5c.
Relativity and Lorenz Transformations are OK and DO play an effect at lower velocities, but until you get to about 0.5c, the time dilation isn't too bad, and you could probably ignore it in most games.
I DO have a book that has all the formulas for Relativity if anyone is REALLY interested. I could dig it out and send it along.
Here is a REALLY abbreviated form:
TOTAL MATH/SCIENCE/ENGINEER GEEK MODE ON
The key formula is the calculation of Gamma, which is:
Gamma = Sqrt(1/(1-(V^2/C^2)))
So, as velocity increases, the value of Gamma increases, but if V = C, then the denominator of the formula becomes Zero and the value of Gamma becomes Infinite (OK, actually undefined, but that is math, not engineering).
If V > C then you get a negative value inside your square-root and things become imaginary (not related to reality).
Gamma can be used to figure out the time dilation of a trip or the relative mass vs the rest mass of an object (it is a multiplier).
So if you are moving at 0.5c the value of Gamma is 1.15
So for every second on the ship, 1.15 seconds passes in the "real" universe. That is only a 15% difference, or about 1 in 6, so not too bad.
At 0.9c, the value of Gamma is 2.29.
TOTAL MATH/SCIENCE/ENGINEER GEEK MODE OFF
If anyone wants more details, PM me.