It is possible to lap the HP seat or VDH has reproductions, I would not buy old stock unless you just "have" to keep it OEM.
This is how I polish them.
You will need a flat surface and some wet/dry sandpaper. A pane of glass works. I use a 12x12 ( 305x305 mm for the metric folks)ceramic tile I purchased at the discount section of a home improvement store. Size does not matter...at least in this case as long as it's big enough....but as always bigger is better.
Start by taping down a sheet of wet/dry sandpaper. I use 800 to 1000 grit to start with depending on the damage. Wet the paper and start polishing the face of the seat IN LARGE FIGURE 8 LOOPS using light pressure and a slow pace. Change the position of the seat in your hand and the orientation of the loops fairly often. The objective is to remove slight amounts of material while keeping it as flat as possible. The large figure 8 loops and frequent position changes are to help even out the material removal. Inspect the face of the seat OFTEN looking for even material removal and that the defects are getting less evenly. Experience will teach you the interval that works for you but in the beginning check often. I use a jewelers loop to inspect the seat. Once the damage is gone, I switch to 1500-2000 grit paper to do a finish polish using the same large slow figure 8 loops.
I also polish out the HP nozzle. I stat by polishing out the interior of the nozzle with strip of purple Scotchbright pad, doesn't take much, just get it clean. Then with a 1/2 in (12.5mm) wood or plastic dowel sanded down to be a loose fit, wrap a strip of the 1500-2000 sand paper around the end of it and using light pressure insert it into the nozzle. With light pressure, make a couple of turns then inspect the sand paper. You should see a light brass ring form on the sand paper and the orifice should be shiny. Again, go easy, the objective to is clean it up while removing as little material as possible. Removing too much is a BAD thing.
Clean everything well to remove any grit and grim.
While on the subject, I test the nozzle assembly before installing it in the reg.
Assemble the nozzle as normal then place it in it's yoke. Install the yoke/nozzle on a tank with at least 1000 psi, open the valve and check for leaks. I use a small pony tank for this since I can dunk the entire thing into a bucket of water to check for small leaks. If you have a slight leak, leave the nozzle pressurized for some time, often times the seat just needs some time to take a set. I usually cycle the seat by installing the pin and pin pad then pressing it in with my thumb. Check again. When you are satisfied that the seat is sealing, use the pin/pad/thumb to release pressure .
This step takes time but it beats having to disassemble the reg multiple times to fix a leaking nozzle.
