Not a good practice to ever reuse any gaskets . Decarbonize with steel wool and kerosene or MMO if you wish , ensure ring end gaps are clean also . Proper torgue and sequence should be via Shop Manual...sorry can't help you there !

(1) = 160 INCH Lb
(2) = Possibly, so long as the gasket material remains intact. But why chance it? Gaskets are not expensive.
(3) = Sea Foam, if used regularly, can help keep the fuel system clean, but the carbon on the heads/pistons is the result of (mainly) the combustion of trace amounts of lube oil. Since the Sea Foam combusts right along with the gas and trace amounts lube oil, I doubt it will help loosen existing deposits. The carbon deposits usually only yield to a good application of elbow grease :^)
E-mail me if you want a service manual for this engine (in PDF format).

Other than curiosity, is there any reason you want to do this? A certain amount of carbon is harmless.

Andy: No amount of Carbon is Useful ! It is a Insulator causing Over Heating at its worst along with Loss of Compression and Bow-by (Ring Fouling ) and Contamination of Lubrication Oils ,especially within 4-Stroke Engines !

Note: Oh perhaps High Compression , High Performance Engines back in the late 70's required a little carbon around the Valves when Lead Free Gas Hit the Market . Only to prolong the inevitable Valve wear due to the lack of Lead lol .

This is a 1992 model and it has issues on start ups, sometimes it spins good and other time it cranks up slow. I'm thinking it has too much carbon and maybe causes high compression, slow cranking at times. Any way I'm giving it to my sister who lives next door, and I have five brothers who I'm sure will say no wonder he gave it to her it a piece of junk. So I want to get it running as good as I can but I'm sure I will still be going over there cutting her grass like I have been doing the last six years, she has no husband.

C: You are correct with Excessive Piston Dome Carbon accumulation it is possible to increase compression to a level where an older starter will have problems turing it over effectively . Secondary concerns with such carbon is serious Hot Spots (Demon Heat) Issues causing a burnt piston. Since you apparently have the heads off follow our collective recommendations . Mownie is most likely correct (usually is) with the Torque numbers , I was going to volunteer 50 ft lbs so we are in the ball park . Contact you Local Jobber and try to purchase Graphonic Gaskets they can be retorqued and have the best sealability of any gasket material , a little more $$ but you will be happy with their performance. Sea Foam and or Lucas Top End Cleaner & Lubricant added to a fresh tank of fuel will finish up the Ring Carbon Removal since you are not removing the Pistons completely for Ring Gap Cleaning also .

***" 50 ft lbs "*** ???
160 Inch pound = 13.3 Foot pound
That would be "in the ball park"?

Mownie : Apparently I was Indicating a National League Ball Park ! 7/16" to 1/2" 4 Bolt Heads call for 50-55 ft/lb of Torgue. Eg: Polaris Tx Starfire Twin Cylinder . Have no idea of the Head Configuration or Sequencing of the Garden Tractor in question therefore I advised that you would have
a better handle on the recommendations for such . Actually never use my Inch Pound Linear Torgue Wrench , perhaps I should dust it off ?