35 johnson overheating

i have a 25 johnson and could not get it runing just right,So i had a 35 johnson that was in good shape so i put the 35hp johnson on the 25hp short shaft, took it out and ran it,it overheated so i checked the pump.the line was not in on the power head,so i fixed that,then i ran it, the pump ran water a min. or so and then it stoped,so i checked the thermisthat and it was bad,so i removed it, and ran it ,water was coming out fine for a min. or so and it stoped, it seamed like just air was coming out,i thought i may have craked some thing in the powerhead, i can take parts off the 25hp if i need to,can anyone help me.thanks for your time.

It would help to post some model #s ????

the motor is j25rcos i had it on a hose with ears.spark look fine

i took the head off,and the gasket was bad so i replaced the head and gasket,the head i put on was used but was good,does anyone know if i check the side covers.