Remove the after market steering wheel and check the remaining wiring.
Horn is a simple contact circuit, you should have a plate and a contact wire (if the plate is gone then two contact wires that cause the horn to sound when put together). Attatch the two to appropriate sections on inside of new wheel.

If these have been removed along with your airbag paraphernalia (assuming the aftermarket wheel required removal of airbag) then im afraid you are probably without a horn.

Stock momo is awesome, why change?