HELP! M turns hard all of a sudden, any guesses?

So, I figgered new plugs, and took Red's many suggestions and got some AC C86 plugs. And she fired right up.

Fed the cows, and shut her down to eat lunch. She didn't want to start after lunch so I could grade the ruts out of the drive. Pulled the new plugs, and they looked like she flooded, dried 'em off and she started fine, tho still turned hard. Worked a while, then went in for coffee.

Tried to start her again so I could finish grading, and had to jump it with the Exploder! 6V battery wouldn't turn it fast enough to start. I almost think one of the cylinders is getting ready to stick. She makes a sound like an old hand pump that can't quite get primed when she cranks. So, I parked her, before pieces started flyin' off.

Any suggestions as to where I should start looking for the problem? She's always turned easy, and started quick before.

Assuming the worst, what engine kit should I look at? Are the Tisco step head pistons as good as the IH Firecrater? She's an ex-dual fuel, and does not have the high altitude head (I live in Colorado).

Of course, maybe I'm unconsciously kinda lookin' for justification for a winter project! It's that suckin' sound that bothers me. That's new.