You will need to redo the drain area in the floor and wall. The only way to do that is demo. If it is not your primary bath..ok. Otherwise..call the plumber.
You will need to have some skill for this type of job. You need to set the flange for the toilet in the correct spot away from the wall and probably have to cut cast iron and attach a rubber boot to plastic fittings. AND have enough room to fit it all in.
Take it slow..ask again or post more pictures and get ready for some gross stuff with bad smells.
PS.. you usually don't need a vent for a toilet fixture, with a 3 inch line.Unless it is galvanized pipe...because they close up to 1/2 inch drain lines.