Will take a look at my Vigor tomorrow, but im almost sure the end of the engine mounting bolts are just threaded on their ends to take a nut. I have encountered engine bolts where the bolt shaft is corroded into the alloy of the crankcase. A total b@@@@@@

Try Plusgas and applying localised heat of the alloy around the bolt. Pouring boiling water from a kettle might free it, or careful use of a blow torch. At worst, you may have to drill it out.