Rear tiller won't go into gear.belt needs replacing

Randy, The gear in the gear box may have sheared a key, or damaged gear. Check the attached links,instruction and guides, Good luck
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Tiller Gearbox Trouble
Murray Ultra Tiller Tines Won Keep Spinning
Update Craftsman rear tine tiller repair stuck gears

If you cant move the tines by hand then I wouldn't think you sheared the pins, more like a gear box problem.

Transmission/differential.
Perhaps the unit sat for 4 years because the tines would not turn and your replacement engine has not solved the real problem.
I'd say disassemble the chain case to discover what's locking the tine drive.

Was is the model number of this tiller?

Go to sears.com. at top of page about center of page find parts. put in model #. It will give you a breakdown of the whole tiller.
R.G.S.

check to see if there is a snap ring on the shaft or a roll pin.

Try rocking the wheel as you try to shift.

That sounds like a "look see". Unless something has wrapped itself around one of the shafts causing resistance (Like a wire, or nylon twine or even certain types of vines...you'd be surprised how well they can hide around tiller shafts both drive and tines causing all types of hidden issues) it may need lubrication. But from here...I'm blind and deaf on this one. Sorry but I'd have to be there with you on one like this.

Little grease on the shaft and some work was able to remove pulley. Found input pinion, tine sprocket and gear all missing some teeth. check with sears and these parts are no longer available. Can anyone tell me where i might find or locate replacement part i need for this tiller?

It sounds like the belt just stretched. Now you need to move the belt tensioner over to put more pressure on it.