Hey Leon there is a product available from GOOD plumbing supplies or a decent welding supply company.
This product is called Black and White and it is torch applied.
High Sulpher content welding/product used with a propane or acetylene torch NO need for oxyacetylene flame as it melts at low temperatures
Fantastic for repairing cast iron radiators and drainage pipe even cast iron under pressure like cast Iron condensate tanks.
If you cant get this try Hercules Pro Poxy as most of the handymen who are not professional plumbers resort to this for piping repairs with decent results.
You honestly should consider rmoving this old section and replacing in kind for a really good job using Mission couplings designed from extra heavy cast Iron to no hub cast Iron piping.
Even a crack on vent pipe could allow dangerous fumes nto the building.
Doing it right is always the cheapest way to go.