It's just a simple matter of calculating three numbers:

Rolling Resistance:

This is generally considered to have a linear relationship to weight. To find the resisting force, you simply multiply the Rolling Resistance Coefficient (RRC) by the vehicles weight. Here's a link to a list of LRR tires to give you an idea:

RRC
Aerodynamic Drag:

The aerodynamic drag is simply a function of CdA and the speed according to the following formula:

Drag = 1/2*rho*V^2*Cd*A

Where:

rho = .002378 (at sea level)

V = velocity (in feet/second)

Cd = (coeffient of drag)

A = frontal area (square feet)

Powertrain Losses:

Typically this would be 15% for a manual transmission.

For this particular case, I'd throw out a guess of 550 lbs operational weight, Cd of .10 and frontal area of 4 square feet. That works out to the following:

Drag = 1/2*rho*88^2*.10*4 = 3.7 lbs

RR = 550 * .005 = 2.8 lbs

hp = force*speed/550 = (2.8+3.7)*88/550 = 1.04 hp.

Factor in a drivetrain efficiency of 85% and you get 1.22 crank hp required.

Since all the above is really a bunch of guesses, I'd say it's easily less than 2.5 hp to maintain 60 mph on level ground.