Like rick said, "Safely, or legally"?
There is an old adage "speed costs, how much are you willing to spend?"
If your wanting to go fast, like most people do when they first get theses cars, the first thing you must figure out is 1.) can I afford to make this car go fast. 2.) can I afford to make this go fast safely. Understand, every system on this car must be upgraded to an equal extreme, not just the engine and transmission.
If your looking for speed, you must first go mid engine There are two chassis options that I know of. (balancing out the chassis characteristics would be your first priority)
1.) 8K for Dave's tube chassis
2.) 25K for Dave's F1 inspired chassis
Second, The next really serious question you must ask yourself, if your pockets are deep enough to make the sterling a true street legal race car, is "Do I have the skill necessary to drive it like that?"
In most cases, lack of driving skill, and less than deep pockets result compromise.
I have landed the Dream setup as Warren calls it. However, despite my luck in nabbing the WRX turbo engine, complete wiring harness, and legacy trans (for less than 2K), I still have to run it in a rear engine setup due to my pockets being less than deep.
Performance building a WRX engine can run you 8-10K (after you've bought the engine), plus the Dyno tune which for me will run 2,500. Then there is the flipped diff (for the transmission), which is 2K. And that's just the motor and transmission. Any thing over 300hp in this car would result in a nearly undriveable car due to its weight.
I have run the figures on the WRX/Legacy setup, with all the gear ratios, final drives, wheel size, weight, even the presumed drag coefficient applied to the equation. The result is a theoretical red line (in 5th gear) speed of 189 mph with this setup.
But now lets toss a little more reality into this equation.
The sterling is a beautiful kit car. Not a race car. They are very rare by comparison to even some exotic cars. Despite this theoretical speed number, this car is not designed for crashes, even with either of Dave's chassis, this would not be a safe car to drive fast, or race. Nor would you want to. The risk of destroying your prize are too great. If you think about it, how many times have you seen a Lamborghini, Ford GT, or Ferrari going down the freeway speeding? Exotic cars, and even kit cars are too rare, and too expensive to do that with (And the exotic cars are built for speed).
Many hours of cad programming and custom designing go into an exotic car, and race chassis. Safety, and stability are the primary concerns.
The Eureka that was being rallied, was from what I heard nearly a 100,000 dollar car. Yet it was still running a VW drive train. If they had run that rally car with a different drive train they could have probably doubled that monetary figure. Yet with all that engineering, and work applied to the car, its chassis characteristics still put into the wall at a relatively low speed like Rick said.
The longer you have your car, you will see your self going into a mental and emotional transition, from speed, to beauty, and care for the car.
So get her running, and safe, take her out and enjoy her. Let people presume what they want about speed and price. All you have to say is "It's fast enough". Let them think what they want.
Once my car is complete and drivable, I may join in some "spirited driving" on occasion but nothing that will endanger myself, or the car.
There is a video on youtube of a New Zealand rally sprint race that shows the characteristics of this car in cornering. The car not only crashes, rear end first, but also flips onto it roof (No way to get out).