Hello Enessr and welcome to the forum. Please feel free to post photos of your progress as you go through the build.

You should try get set up so you have access to the online shop manual as it explains exactly how to set the crankshaft in the case, where to apply triple bond and the torque sequence for all of the bolts.

I have never figured out what that little channel is as it doesn't go anyplace and the mating surface on other side is flat. Personally I've never filled it. I generally just use a clean finger to dab triple bond on all the mating surfaces ensuring a nice even coating. Not too much and not too little.

I'm pretty sure no one on this forum would endorse a Chinese after market cylinder head as you have no assurance on the quality. I personally have VHM heads on mine (300 kit) but I realize they have a bit more cost. I think you would be better off looking for a used upper Honda stock head and put it on lower to get the center plug. Be aware you will then need to use a "shorty" plug and cap to keep the front wheel from hitting the plug. Some might say you could get away with a stock plug, but I know on my bike it just kisses the plug cap when forks fully compressed so I use a shorty plug. They are getting hard to find and are pricey.

As for the clutch, when they start to look like a steak knife its time to replace. I help an American kid racer and part of my help is to keep his Moriwaki MD250 practice bike running. Last year he was convinced the bike was about to nuke on him. I found the outer clutch basket looked like steak knife on the edges. The clutch was hanging up on him. I replaced it and bike was back to normal. I think you can still get new Honda outer basket but the inner is hard to find. T2 Racing may have an after market version which would be good quality.

Good luck and hopefully I've given you some useful info.

Eric