The lapping sticks you have are great but that paste is absolute rubbish, I have a full container of it, tried it on one valve and put it back on the shelf. It dries too quickly and is way too coarse for my liking ( I know it has coarse and fine paste ). Try and get ARBO brand, it stays wet for a long time and you can see how well the valve and seat are mating together while using it- kind of like bearing blue - once you start lapping valves this will make more sense as you get a feel for it. It leave a nice finish too. By all means give it a go but you may find it frustrating to use. Mix it up well with a screwdriver before trying to use. Another thing you should buy is a straight edge if your going to do more of these engines. You need to make sure the heads are flat. Any more than 2 thou dip or warp and you should look at getting skimmed at the machine shop. I usually check them 5 ways , front to back on the exhaust side of the chambers, again on the inlet side then again straight down the middle. Then diagonal from front left to back right then front right to back left. Buy a nice long proper straight edge, mine is around 1200mm long and don’t drop it!