Having some Honda problems any ideas?

I was riding my 2006 crf 250r and it just had a top end rebuild and it was running fine for a while but I just took it out a couple days ago and after riding for about an hour the compression in the kickstart got really stiff and a couple times it backfired when I kick started it and I couldn't start if at all. Any ideas what it could be?

Add an aftermarket exhaust without up-jetting or have a bigger bore without up-jetting?

Usually the back-fire pops are from running lean, so check the burn on the spark plug tip, it should be tannish, white or gray is lean burn.

Check the oil level.
Low on oil will starve the top end of lubricant.
Remove cap #13 while the engine is running to check if oil is reaching the top end.
Oil should splash out of the hole.
http://www.partsfish.com/oemparts/a/hon/5053e0b7f870021c54be0be0/cylinder-head