I'd go this way myself. It sounds like it sat empty for awhile and the devider wall rusted through.
To run straight premix, unbolt the oil injector and disconnect it's drive shaft. Then bolt the injector back down.
Now put a tight sealing rubber plug over the intake's oil connector, loop the existing hose from the oil injector's output to it's input and cap off the oil tank output connection. This last one's important too since obviously the gas would otherwise drain onto your exaust.
After that you just need to mix the right ratio of oil to gas and you're set.
Myself I like the oil injector though, so I'd probably get a new gas/oil tank off ebay, clean and kreem it for durability and go with that.