Given that amount of power, my guess is that you'd top out around 40-45mph.
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
Plugging in some numbers I ballparked (4000lbs, 35 sqft frontal area, 0.45cd), you would need 23HP from the electric motor to maintain 55mph on a level grade, which is something like 18,000 watts.
Even without considering the electrical part of the system, at that point, you're getting into the price range where a new-to-you low miles gasoline or diesel engine will be less expensive and likely a lot more reliable.
I would however like to see something like this built.
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Some back of the envelope calculations:
Let's be optimistic and assume 90% efficiency from your A/C motor (most EVs are DC, so I doubt you'll find one from a wrecked car). You'd need about 20kw to cruise at 55. I don't think you'll find too many in this size that run on gasoline, but let's assume you score a diesel one for not too terribly much:
Generac Power Systems | Home Standby Generators | Protector Series | 20kW | Generac Power Systems
^ The manual claims approximately 2 gallons per hour @ 100% load (55mph), which would put you in the ballpark of 27 miles per gallon on diesel.
If you can find a gasoline generator of this size, and assuming it's just as efficient, due to the reduced BTU content of gasoline vs diesel, you'd be closer to 24mpg - which is still a very attractive number in a vehicle that size.
However, I'm willing to bet you can get 24mpg with a drop-in diesel engine and some taller gearing, for a lot less than the cost of a 20kw generator + A/C motor + controller + fabricating mounting and drivetrain components.