Lapping stainless steel valves??

I would slide a piece if fuel hose over the shaft then chuck it in a drill. Then I'd spray wd-40 on the valve and hold it with a scotch brite pad and pull the trigger. Worked like a charm.

If the seats and valves have been machined correctly then you shouldn't need to lap them, I've never bothered doing it. If they're worn, replace them. If they leak with new valves or seats then complain to the relevant party.

Lapping stainless is fine. Ti is a no no.

It helps to ask from people who do this for a living instead of the crazy suggestions you get on here...

It's always a good idea to have your seats cut if its available to you.
If not,
Go get yourself some "fine" valve lapping compound and put a little of the valve face and spin it against the seat. This will help mate the valve to the seat. If you cut the seats this is not needed as the fresh angles will wear to the valves.

Lapping the valve will also let you see what part of the valve face is sealing against the seat. This will tell you how much you have to work with on your valve seats. If the majority sealing surface is on edge of the valve face you will start wearing out valves with the quickness.

Judging from your pictures, your old valve looks really good minus the sludge...it's just sealing really high on the valve face. The stem looks good though.

Couldnt a person use a new ss or ti valve for lapping and swap it out for a new non lapped valve.

I use a set of Neway valve cutters to condition the seat if it needs it, check that before you do anything. If your seat is cupped then lapping will do no good.

If your seats are in good shape and you would like to do a small amount of lapping to be sure you have a tight seal, here is what I would do.

remove valve stem seal and place the new valve in. Turn the head upside down so you are looking at the bottom of the valve. fill the valve area with a small amount of water so the cup is submerged. Blow a small amount of air up and under the back of the head to see if you get any air bubbles. If you don't, I would install new seals and go ride.
If you do see some bubbles, mark the sealing face of the new valve with blue or black sharpie and do as charper has explained. Remove the valve and check frequently to make sure you are getting a nice even mark across the sealing surface of the valve face. You will be able to tell if you see your marker start to disappear evenly.

Once you feel you it is as close as it can get, clean up and go ride.

One important measurement that is often over looked is valve protrusion. Make sure you measure the old valve spec and compare to new valve spec,(its in the manual), the reason to check this is you dont want the valve protrusion at the wear limit before you start lapping. If it is and then you get it tighter, you run the risk of having the cam lobe push done a hair out of spec and damage valves. I couple of thous probably wont destroy anything, but remember a couple of thou is the diff between out of spec and metal against metal!

good luck-