"...think any of the others may have streched, and if I should just replace them all?"

If they haven't broken, you're probably OK. Loosen and retorque 'em all to 15 lb-ft.

"...it broke half way off so I can still get it with a pliers or some sort of tool..."

You're very lucky. A pair of Vise Grips whould work fine.

"Maybe my honda lawnmower? or the huge chevy van in the driveway?"

The Honda will have metric fasteners, the Chevy won't.

"...that has a 305 that sucks a little more gas than the 745..."

Yeah, but if it's registered and runs, use it for a day or two 'til you get the correct parts.

Remember, in a pinch you could buy an ordinary metric bolt at the hardware store. That application's not demanding.