The spine on a trowel is a cast alloy or aliminium? ... is it not? I've had this talk before with someone that knows about these things and was told it was an impossible weld? If you can weld them, put the rivet area in a vice or G clamp under V tight pressure and then apply the weld centrally to the tip and I'd think carefully before I went either side (in case it warped) You also have to be careful that the welding thingy doesn't put a hole in the blade.
If you can't weld, I'd try this way:
> Drill the old rivet out,
> tap the hole in the spine with a thread,
> Slightly counter-sunk the rivet hole on the working side of the blade without enlarging the hole,
> Now is the hard bit. You need a bolt of the right thread with a counter-sunk head,
> Put the bolt thru blade into spine hole, tighten and grind off the surplus head of the bolt. You could also trim the bolt if it protrudes from the spine.
I once had some cxxt in a telehandler put a tub of mortar down on a 1st floor scaffold without asking ... right on top of my tools and of course, a brand new 'first day on site' 14" marshalltown!!! It pushed the rear of the handle down, spintering it and snapped the spine. I used some plastic 'metal' and bedded metal splints in down either side of the break in the spine. It worked but in the end I cut the rear half of the blade off and made a good margin trowel type of thing. It was great for getting in behind pipes, radiators, etc.