Need to do a measurement of the chamber as well as the bore size to know the size needed for 'shells' and wads.
For me, that would be a first step if I decided the gun was suitable for shooting.
The pressures generated for a BP 8-bore load will be quite low.
Brass cases can be turned, however, perhaps the chamber is meant for much thicker paper cases.
Loads are likely printed in Grener's "The Gun and it's Development" 9th edition.