If its getting really hot it sounds like you need a external ballast resistor. The old coil may have had a internal resistor. I know mercruiser used both type of coils.

I know my buddy ran his without the resistor and it died after about 40 min. on the water.

This one here needs external resistor.
http://www.iboats.com/mall/partfinde..._id.189688285/