Classic Montauk's with a 2 stroke Mercury 90 hp report WOT speeds of 42-45 mph. Your boat's performance is horribly wrong. There are several intertwined factors that could be the cause:

Carburetors tuned- If your carburetors are not clean and synched your engine will not perform well.

Propeller - If your engine has the wrong size and pitch prop, it will not perform well. You want a prop that will reach 5500 rpm when running lightly loaded, full fuel and one person.

Mounting - If the engine is not bolted on the transom at the right height your engine many not perform well.

It is important to work on 1 variable at a time. First, check the prop. What is the make and size (part # stamped on the inside of the hub)

Are you on a lake or the bay/ocean?