Help in taking engine out of Yamaha Neos 50

Hello all,

My lad had a nail in his tyre so we had to loosen the exhaust, unfortunately the nut stuck on the exhaust stud and instead of just turning it out it snapped the edge of the exhaust flange ! I'm going to try and weld around a new bolt to hold it in the flange but if this fails it will be an engine out job to replace the barrel. Is this just a case of unbolting everything and sliding the whole assembly out the rear of the bike ? Is it worth going for a 70cc kit if I have to do this ?