There isn't much engineering involved.
The sportster has only gone through 3-4 major changes in the nearly 60 years its been in production.
People may sneer at it, but if they can sell every single copy they make year in and year out without much expense on R&D for nearly 60 years, why fix what's not broken?
FWIW, Honda makes a very close facsimile of the bike and that sells well too.
HP? *shrug* For about $2000, you can bore the jugs out, add a set of Buell XB heads, put a better exhaust on it and make 100 hp, if that's your thing.