Cast iron downpipe access hatch repair

Hi,

I've got a cast iron downpipe which has a section with an access hatch. The bolts on the hatch door have been corroded over time and have broken off, so it leaks quite a bit in the rain. I've tied it back on for the moment with some string, but looking for a better fix without having to replace the whole section ideally. Any ideas?

Cheers!

Sand back to metal best you can
Clean with meths
Black polymer mastic to stick cover on

Drill and re thread the holes , then new bolts . Pretty straightforward