Picture says a 1000 words. More useful advice can be given with a pic for us.

Ok here is a photo of the old hardware, I thought I wrote 1000 words or so!? :)

i added a board and fixed the existing latch crossbar to that as a temporary measure but I still want to remove the old hardware.

For removal, you will need an angle grinder, metal cut off wheel, face shield, long sleeve and gloves. Cut off the rivets from the side, then anything else that protrudes once the latch is off. Follow up with a grinding disc using the same angle grinder to make it smoother if needed, also can be used on the blob you describe.

For fastening, a new sharp drill bit and WD-40 or equivalent will do to make the holes. Then use the correct screws designed for fastening to metal, I would say pan head self tapping screws. Might be able to get away with not predrilling with the self tapping screws, However, for correct predrill hole size see the charts here.

Ugh that's what I thought - buying an expensive new tool to do this job. Maybe it's time to hire someone to do this!

Although an angle grinder is the simplest and most direct method, consdiering the specific use there are other methods to get the job done. For example, use a centerpunch to make a dimple in the rivet head and then drill them out.

A hack saw would cut the rivet heads off, too.

A dremel tool with a cutting wheel will also cut the heads off. Dremel with a grinding head will also smooth out the old weld blob.

Dremels can be had for ~$50 and might be a lot more use in the future than an angle grinder.

You could see if you could rent an angle grinder.

I agree - A cheap angle grinder should be the fastest tool for the job, but one of the basic Dremel models would also get the job done, and would probably be more useful for other work.

Bruce