He's using high heat BBQ grill paint already.
The highest commercial heat resistant paint will be exhaust ceramic coating paint.

VHT SP102 FlameProof Coating Flat Black Paint Can - 11 oz.

https://www.amazon.com/dp/B000CPJLGM/ref=cm_sw_r_cp_apa_6OqrxbC9N3WY3

VHT is popular for heat/flame resistant paint.

• Here is their primer
  
  
• Here is their colored coating
  
  
• And they have a satin coating
  
  I think proper cure procedure is idle 20 min then cool 20 min, idle 30 min then cool 30 min, then run it normally for 30 min or so (ride it around). You can also bake it in the oven to cure it if the muffler on the bike won't get hot enough.
  
  Some people have very good results with this paint. I think it's very sensitive to prep and application. Have to lightly scuff or sand the surface. Then thoroughly clean with isopropyl alcohol or something to get all the dirt and grime off. Start off with a couple of light coats then start spraying heavier and heavier.
  
  I have not tried this myself but I did order it last week. I removed the mufflers on my bike and I plan on painting the underpipe section.

probably something like this: https://www.amazon.com/VHT-SP102-FlameProof-Coating-Black/dp/B000CPJLGM

This.

I recommend VHT paint. I painted my car's turbo manifold, turbine housing and downpipe with the shit and after about 20k miles it still looks good.

• Painters tape
  
• plastic drop cloth
  
• Scissors
  
• VHT flat black flameproof spray paint

Took me forever to figure this out. It’s not easy finding parts for the little three inch pipe. I ended up using the transition plate listed below with high temp insulation between it and the roof. Then on the roof I installed one of these high temp silicon rain guards. Also, if you are looking for a chimney cap Home Depot sells a cheap one that was recommended to us by cubic mini. We painted the stove pipe and chimney cap black using high temp paint.

176203 3" Ceiling Support/ Firestop-... https://www.amazon.com/dp/B002NHWAQK?ref=yo_pop_ma_swf

Insulation, Wool, 0 to 1200 Degrees F https://www.amazon.com/dp/B01LZI4T1U?ref=yo_pop_ma_swf

3 Silicone Hi-Temp Pipe Flashing... https://www.amazon.com/dp/B0792PT94W?ref=yo_pop_ma_swf

https://www.homedepot.com/p/100396919

VHT SP102 FlameProof Coating Flat... https://www.amazon.com/dp/B000CPJLGM?ref=yo_pop_ma_swf

I honestly have no clue about this. I'm just using google. Something like this is what I'm thinking https://www.amazon.com/VHT-SP102-FlameProof-Coating-Black/dp/B000CPJLGM

if you're doing your rims, i suggest ceramic "header" paint from the auto parts store. I painted my truck rims matte black 10 years ago, the paint still hasn't chipped off. http://www.amazon.com/VHT-SP102-FlameProof-Coating-Black/dp/B000CPJLGM

Header paint is probably your best solution out of a rattle can.

High temperature powder coats may be an option,

Ceramic coatings work, but they're pricey

Regardless of what you do, I'd fire it before cooking with it to drive off any chemicals.