HVAC over 25 years.
New gas valves are proprietary, old ones are not 'usually'.
Still, if you don't know, you shouldn't be working on it. It also make a difference whether it is propane or Natural Gas.
Knowing you will work on it anyhow...
1) Make sure gas and electric is shut off
2) remove gas valve and thermocouples
3) cap off gas lines DO NOT LEAVE THEM OPEN
4) take gas valve, thermocouple, and all the data on the furnace to an HVAC supplier and get a replacement. Note Propane or NG and voltage. Some gas valves are universal replacements and need to be further set up in the field per requirements. Also note - most will not sell to you if you don't know what you are doing.
5) Install suitable replacement reversing the procedure using gas teflon tape (yellow) and only black pipe (No galvanized)
6) clean the heat exchanger and check for leaks (soap bubbles on the joints)
7) check for back drafts (usually indicates a leaky heat exchanger) - a cheap and dirt way is see if cigar smoke sucks up the chimney from the burner area with the fan running
8) adjust/check flame using air intake to each burner - flames should burn blue with the tips licking yellow
9) adjust/check thermocouple flame - flame should surround couple tip, if not the orifice may need cleaning and/or pilot gas pressure adjusted
10) once you are absolutely sure everything is back where it was when you started WITH NO LEAKS, fire it up.
11) MOST IMPORTANT skip steps 1 through 10 and hire somebody.
12) clean the filter and oil the motor
PS many high efficiency furnace do not last. Only 80% of Natural Gas you buy is actually combustible. To make a 90% furnace you must burn part of the inert 20% - some of that is air (oxygen) and hydrogen. When you oxidize hydrogen, you guessed it, you get acidic water which is corrosive. I've taken high efficiency furnaces apart where the combustion fan had completely disappeared! I personally believe in middle efficiency for this reason. If you do buy a new furnace, buy the name brand with the least number of proprietary parts. Most brands are made by one of about four or five companies.
