Re drill the existing hole larger, then fill it with thickened epoxy.  After the epoxys hardens redrill the hole to the correct diameter.  This will leave a doughnut of hardened epoxy that will seal the edges and strengthen the crack..

   Thank you Jackie.  Would you recommend re-varnishing over the crack too?

With a crack in hardwood, your main concern is strength and not water penetration.  The drill, fill, drill fix will prevent water penetration but will do very little to restore the strength lost to the crack.  If it were mine, I would drill two holes top to bottom and through bolt with stainless hardware.  If you use pan head bolts and countersink, the repair will barely be noticable.  I'd do the other side also so it looks like it was part of the plan. 

If you do not want the hardware to be seen, then you could add a layer or two of fiberglass tape wrapped around the end.  This will be more work because you have to remove the varnish first, add the glass, add fill coats, sand fair, then re-varnish.  

Just make a new tiller.  Or cut the end off of yours and make a new end, if you can live with it being a few inches shorter.   

   +1 for fiberglass

Or instead of a bolt, you use a small trunnel. Not sure of the dimensions of your tiller, but maybe an 1/8 inch dowel would be appropriate, and do it on the other side as well for strength and make it look like it was the original plan. 

Glued in with epoxy, it would make the tiller a lot stronger and look cool!