I have been coming accross these Heil brand gas furnaces that do not have the MCA and MOCP on the nameplate. They list the maximum input current which I think is what the unit is rated. How do I calculate these values.

This usually applies to HVAC compressors. Does this furnace have anything more than a blower, igniter and some controls?
When in doubt use 310.16 and 240.4(D)

There is also a small combustion blower.

Look at the manufacturer's installation guide for the furnace. Usually they will tell you what size circuit. Some even tell you what size wire to run and depending on the length of the run they may have you increase to the next size wire. I'm seeing some of your larger furnaces specify a 20a. circuit.