Just a bit of an update. I purchased new rubber, front and rear, for the J300 and set about fitting the tires myself. The front was no problem, once I figured out the best method of breaking the bead. The rear however is a different story. It was all going smoothly until I tried to remove the subframe that slides on over the right side of the axel. I saw a few mentions of it being difficult on some older threads here, and they we're not wrong. It's not coming off, no way, nada, no hope in hell. I tried everything eventually resorting to a propane torch with the hope of enough heat expanding the metal and breaking whatever bond is holding it in place. Still no joy. I had to admit defeat and put the bike back together with the old try still in place.
Today I rang a bike tire shop to see if they would do it and had an interesting conversation with the owner. He was quizzing me about weather the tire has ever been replaced before, obviously trying to preempt this exact issue. When I told him about my struggles he said I had to take it to the local Kawasaki dealer as this was a problem on many of these early models. Mines a 2015. Apparently he's run into this issue a number of times and had to refer the bike to the dealer who has to apply so much heat to get the subframe off, that it destroys the bearings. It sounds like Kawasaki was covering the cost of the removal and bearings a few years ago, but I'm yet to confirm that. I suspect however I'll struggle to get anyone to cover the cost at this point in time. Got the scooter booked in for next week, so hopefully the dealer has better luck than me.