These engines are a little bit difficult to pull start, you have to pull them with authority and you need your implement on the front and sometimes it needs to be against something so the pull rope doesn't pull the whole tractor around when you pull on it.
If the above sounds like what you are going through, I would say your exhaust valve is good, you have good compression.
Don't be afraid to mess with the main jet adjustment. Get it all warmed up, and rev it up high, it would be preferable to have the rotary mower on it and engaged to present a load. With it running like that, turn the main jet out till it starts stumbling and running rich. Then turn it in till it just clears up. It will run the best like that, but it may be a little bit stinky, you get that exhaust smell in your clothes after you are done using the tractor. If that bothers you you can lean it out just a little bit more to get rid of the smell, but I have found out in the winter in cold weather, it runs better moving it back out just a little bit.
You can engage the pto without it grinding. I am surprised more guys on here that are particular about their tractors don't do this. I do it because I do not like the grinding and crunching metal when I turn the pto on.
1. Put the high low speed lever in the middle in neutral. Then lightly push the forward reverse lever to forward. When the tractor is cold, you will find with the high low in neutral, the tractor will move with authority and usually in high when the forward lever is pushed.
2. Move the tractor gently toward a large object that doesn't move, and you do not care about getting a little scraped up. This can be a old building or a tree, old stump, etc. With the high low still in neutral, push the forward lever till the implement gently touches your holding object, and then press down farther on the foward lever. You will hear the engine load down a little bit, while holding the forward lever so the tractor is pressing against your object, push the pto in with your foot.
3. If you lightly press the pto with your foot, while pressing on the forward lever, you can feel the pto speed slowing down when it's speed is slow enough the pto lever will drop right in.
After you used the tractor and the oil is hot, usually you do not need the large object, just put the tractor high low in neutral and then engage forward the tractor usually does not move. The pto will go right in no grinding. What you are doing is locking the high low planetary together with the forward and reverse planetary. Since the tractor can't move, the forward reverse planetary will stop the high low planatary, which are what is causing the grinding.