The UCE has about 30 foot pounds of torque.
Assuming it could hang onto that torque for several thousand rpms, then it could reach 40hp at 7000 rpm.
(30ft-lbs x 7000) divided by 5252 = 39.98 hp @ 7000 rpm.
Considering it already has an 8.5:1 compression ratio, it won't be able to withstand much compression increase, so that's pretty much out for torque increase.
Stroke is fixed at 90mm.
Bore may be able to be enlarged.
A properly tuned exhaust might add some.
I don't know what the port flow is, or whether it could hang onto the torque up to 7k rpm or not but I kind of doubt it. And the hydraulic lifters would probably pump up like they always do and cause issues over 6000 rpm. And we still have the long stroke piston speed issues. And we have the previous history of India-Enfield doing some over-rating of the actual power, so we don't really know if it produces exactly what they say it does in stock form.
So, realistically, without a bore increase we're looking at a potential of:
(30 ft-lbs x 6000rpm) divided by 5252 = 34.27hp @ 6000 rpm.
If we can bump the torque peak with a bore increase, then we would get a little more.
If we could get some cams in there, and dump the hydraulic lifters, or put in a a big cam with hydraulic roller lifters, then we could see some more come out of that, but probably with a loss of some of the lower end power.
That's a "quick and dirty" assessment of what we're looking at for potential.
So, with a weight of 370 pounds, and 34.27 hp, we see a weight-to-power ratio of 10.79 to 1.
Contrast a stock Harley 883 with about 50hp and weight of about 570 pounds, and it comes out to 11.4 to 1, but it does have more torque than the Bullet, so it will be able to deal with the extra weight of the rider better.
So, I think that a modded UCE should be in the ballpark with a 883 Sportster for riding, and should be able to keep up with it pretty well. I don't know if you could beat it, but I think you could hang in there with it.