The thing you will really want is not horsepower but torque. They used to say in Detroit, "Horsepower sells cars but torque wins races."
The reason for this is, peak horsepower on any engine--and especially a very small engine like a 1600--has to come at high rpm in order to be impressive. (For a modern example of this phenomenon, check out the Honda S2000, which I believe was built with a goal of sporting 250 horse from its tiny 2 liter engine. Honda did it, but the 250 horses come at something like 8300 rpm. The engine makes something like 150 ft-lbs torque and that's at maybe 7500 rpm.)
This means that, to get full enjoyment from the car, you've got to rev it to the stratosphere every time.
Note also: the difference between the 7500 rpm torque peak and the 8300 rpm hp peak is only 800 rpm. That's called the "power band" and it's not a lot of wiggle room.
Bigger engines tend to give you a lower rpm torque peak and a wider power band. That makes your driving experience much more fun.
Running at lower rpms tends also to make your engine last longer.
So, in your example, building a 1600 Type 1 to 90 horse is doable but, best case scenario you're going to get engine that makes 90 horse at 7000 rpm and pulls peak torque--maybe 80 ft-lbs--at 5500. You'll be able to tell everyone "it's got 90 horsepower, just like an old Super 90," and show them the dyno slip to prove it. But you won't be able to pull away from anyone at a stop light.
Or you could build a 1915 to make 90 horse at 6000 rpm and make 85 ft-lbs at 4000. That would give you roll-on power when you're cruising down the highway at 75 and still want to pass someone. The greater torque will extend down towards idle too, so you'll be able to take off from a dead stop with much more authority.
The 1915 will not look any different from the 1600, btw. You could tell people it's a 1600 and they could never never know you were lying unless they pulled the heads off and mic'd your cylinders.
Of course, 90 horse isn't ver much either, and with a crankshaft change you could get even more cc and more low end torque, and the engine will still look like a 1600.
This is why so few of the members of this board run stock-sized engines.
