That old carbon should come off the valves OK with a rotary wire brush. 50 years ago I held them by their stems with 2 wood blocks (think: paint mixer stick, broken in half) and cleaned them with a wire brush in my trusty Black & Decker variable-speed electric drill (still have it!), then relapped them in their seats.
The stubborn carbon in the chambers will yield to a Swiss Army penknife's curved blade, nearly a perfect curve match (I have about 6 of those).
Sorry about the master cylinder's bolt, but you are not the first to report that - and not just for the Hondas, either . 
I think "4into1.com" is crypto for "parts that failed inspection, sold anyway"...
so, safe to use some steel on the valves, as long as I don't go ham? Was the rotary wire brush you used brass or steel?
yeah, no more 4into1 parts for me!!
edit - I tackled the intake manifold with the dremel as practice before doing the porting. It was more forgiving than I expected and I got a good feel for it. I did both where it connects to the head as well as where the rubber boots connect. I used the cone shaped bit, a sanding flap wheel and a maroon scotchbrite sponge bit. Its a significant improvement! I'll do a modest bit of porting with my new dremel confidence, staying well clear of the valve guide later this week. My exhaust port has so much carbon buildup, I'm not sure where to even begin. The head does not fit in my ultrasonic tank either, so I suppose I'll have to do a little work with a few brushed by hand and some PB Blaster. I do plan to wash the head in the sink with soap and water once I'm all done cleaning up the ports, chamber and gasket surfaces. I don't believe there is anything on the head that will rust, right?
I'll also go after the valves and chamber with a steel brush, knife and/or wire wheel. Oh yeah, and I've been chasing the threads on all the bolts and nuts, and soaking all the pieces in Evaporust and giving them a little WD40 misting. All my hardware should be good and refreshed for reassembly.
I forgot to ask this before, but does it matter if the valve assemblies go back where they came from? I labeled and bagged everything so it all goes back exactly where it came from, but wasn't sure if it mattered. I would like to throw all the valves in the ultrasonic cleaner, but they will get mixed up that way.
Before/after:
