Well, given those decals looks like it's a Clark 667!

Looks original given that those decals are still there, which implies it's the original paint, pretty neat! I personally don't know much about Clark Mfg. Corp, I know they used to make some race karts back in the 60's, and apparently they later built yard karts like your 667. From the looks of it I'd say that's a late 70's/early 80's yard kart.
Looks like it originally had a jackshaft setup since it has the sprocket on the right, probably a plain 3 or 5 HP Briggs flathead originally. Do I spy a differential axle back there?? Some more pics of the axle right under the engine plate or from behind would be great.
As far as value, I'd say it's probably not worth much more than what it'd cost to get it running, depending on where you live. Try looking for other karts on the local Craigslist and see what they're selling for in similar condition. Around here in Texas I'd probably get $200 or so for selling that kart, but value varies a lot by location.
My recommendation is to fix it up and get it running good for your grandson, you two will probably learn lots of stuff in the process, and all the fun he'll have with that thing is worth so much more than cash. Plus, it looks like it's already in great shape for its age!