Have an older model Tappan furnace. I have

You will need a Honeywell universal thermocouple most Hardware stores have them Home depot etc or a plumbing supplier

Sound like air flow problem to me, the inside and the outside. check the inside filter to see if it's clean and clean the outside condenser coils by washing it.

older standard efficiency furnaces have a thermocouple. mid and high efficency models do not. they are electronic ignition with a hot surface igniter

should be same as old one was.

usually thermocouples are universal, the main difference is the length of the lead which doesn't really matter as long as it is long enough. You can pick them up at a hardware store usually located in the plumbing department close to the gas water heaters.

Replace the thermocouple.

Try this, do you have a butaine stick lighter? Try to heat the thermocouple with it ... see if you can get the main burner to come on. If not - definitely, replace the thermocouple.

Thanks for your question.

almost all thermocouples are universal just need to know the length, ie. 18" , 20", 24" etc...

Here is what I think you are looking for:
18 inch thermocoupler: G1019418
here is a good website
http://www.whaleyfoodservice.com/garland-parts.asp

Good Luck!

Relighting Your Furnace's Pilot Light
- Turn off the gas at the main valve at your utility box - Wait five to ten minutes for gas fumes to waft away - Get a long match—yes, a fireplace match, not a standard match - Set the gas valve on your furnace to "pilot" - Press the red button as you hold the match to the pilot valve - Keep pressing the red button for 30-60 seconds so the pilot can heat the thermocouple. Release the button and see if the pilot remains lit - If pilot goes out, wait five to ten minutes and repeat the process - Once the pilot stays lit, turn the main gas valve back on - If the pilot light continues to go out, call a technician, who may need to adjust or replace your thermocouple or adjust your pilot

That information is usually on the data plate on the furnace and on the motor itself. All you need to know now is the HP. Other than that a universal consolidator motor is an easy fix too. It can handle 1/6-1/3HP.