Are 90 hp for a car engine enough to drive on the highway?

In late '60s I had my funnest car, a Austin Healey Sprite -- 46 bhp.

In early '70's I helped drive my sister from Detroit to Los Angeles -- in a 36 bhp Volkswagen aircooled beetle

In early '70's I a '72 Pinto, city freeways and Interstates on camping trips -- 86 bhp.

In late '70's I had a mini camper van (5,000 pound GVWR), went to Mexico to Canada -- on a 43 bhp Fiat 238.

(although it was underpowered and only did 57 mph on the flat)

Today, retired, can buy whatever I want, I have a smart Fortwo convertible. Did just fine doing 80 mph to Phoenix and back -- 3 cylinder 999cc 70 bhp.

That Suzuki Swift is overpowered and can do over 100 mph.

That's big. The Metro have one solid liter. I doubt you would exceed the speed limit, but it kept up as well as any of them up to 75.

Its strong enough but you can add performance things to it like a better exhaust, higher performance spark plugs, better tires etc.

Modern small engines are perfectly capable of highway work. Even the budget versions have at least a four or five speed transmission nowadays, which helps a lot. Accept you may not have too much extra acceleration at highway speeds, and that you may lose some speed on hills especially if you are heavily loaded and carrying four super-sized passengers etc. Other drawbacks; the motor may feel "busy" and the ride may not be optimised for cruising, but you won't really notice. Bigger cars are roomier, more torquey and compliant over bumps, but they are also carrying more weight and they are not usually significantly faster and they lose out when you reach urbanised areas. Horses, courses.

You bought this thing 6 months ago and haven't taken it out yet?

there should be online info re: the top speed etc for that vehicle or in the owners manual