At the top on the right hand side there is a search bar ,you can find info there on just about any engine.When you do enough to the engine it could rev three times what it could when it was stock. That will make the flywheel explode and that could kill you or someone else,that is why you need an aftermarket flywheel and it also gives you adjustable  ignition timing ,you need the flywheel if you are going to race it,they won't let you race without it Marc birdman exprss who is a moderator on this site is also an Arc dealer ,he can get you the parts you need,you need an aftermarket connecting rod too,the stock one won't take much RPM

I forgot to say the 38 block takes a crankshaft with larger main journals too,so you need more than a block ,maybe you can find a whole used engine to rebuild

Get the 38,there will be no sleeve left if you bore the 35 that far and use the 23 HP heads,the ports are much higher than the 35 cubic inch heads and they flow lots better That is probably a Quick Way boring bar and if it is in good shape it might be a good deal but if you have never used one it might not be something you want to use without some experience

When Ryan Kerr raced with the Kohler he used no flywheel at all ,The only thing he had was a piece of quarter inch aluminum with magnets to trigger the ignition .When he raced in Delaware Ohio  2008 and won the finals I was talking to Bobby Clevland and he wanted me to show him Ryan's mower .When I told him about the no flywheel he wanted to see what it was about,I don't think he believed me.

It is too bad you don't have an overhead valve engine  like a Vanguard .When I helped Ryan Kerr in 2008 he had been racing all summer and ruined most of his engines but he wanted to go to the All American so we had to get an engine together (he was using 38 cubic inch Vanguards) He only had one head that was usable and the other head was smashed and he said it was junk.I welded the head and put new seats in it,soon it was good again,I had some Toyota intake valves that I put new keeper groves in and shortened,they were a lot too long .the engine ran ok and would rev 13000 He went to the race and won in FXT  He said it had lots of power and said he only drove it hard was at the start of the race and on the restarts .He said he could have lapped George Herron but he didn't want to get him upset
We didn't have low compression,it was 14.5 to 1 and had about 80 HP

If you thought you were going to have low compression,my brother has low compression,he restored a 1942 Harley 45 cubic inch motorcycle ,he said it has 23 horsepower and is between 5 or 6 to 1 compression ratio. I might be getting old but I don't want to ride something that slow

A three angle valve job is the only right way to do a valve job and it can help a lot and it is the only way to move the seat to the outside of the valve face where it should be and to narrow the seat when it is too wide . Just doing a good valve job you will be able to feel that the engine has more power.I have never tried to weld a head on a flathead but have welded lots of overhead valve heads to make more compression and sometimes to repair damage. Once you have moved the seat out on the valve you can backcut the valves to make them lighter and flow more.

It really doesn't matter what size the pilot is as long as it fits tight and can lock in the guide. It is an air cooled engine and the valve needs more clearance than a water cooled engine.I rebuild engines and have worked on thousands of heads. I have used those cutters but I usually use stones and Sioux seem to be the ones of choice. they seem to be the most accurate .