Dont cheap out - if the rear is leaking the front is not long lasting as well do the both and have the valve seals replaced as well - we end up re freshing the top end when that happens as you will be in it again soon if you dont
I agree with the previous poster. It's not a difficult job to do with the motor still in frame, but since you have to take the carb, manifold, exhaust, top motor mount and head off anyway, you might as well pull both jugs and at least check everything out. You might not even have valve seals in those heads. They didn't come from the factory with any, and the stock guides have to be machined to accept seals. But it's worth checking, along with rocker arm fit and condition of the pads where they wipe the valve stems. Check out the bores, too. A new set of Hastings rings aren't too pricey, and you could probably rent/borrow a ball hone from an auto zone type place if you don't have one. A lot of these places charge a rental fee when you pick up the tool, but give it all back when you return it. A light crosshatch in the bore is all you need if the cylinder isn't scarred or worn enough to require a re-bore and oversize piston. Of course, if it ain't broke, don't fix it also applies. To me the worst part of top end work like this is cleaning all the gasket surfaces and getting the pans to seal. Factory OEM pans are fine, but often the aftermarket pans have a wavy surface and have to be rubbed on a piece of sandpaper over a glass plate until flat. Paint the surface with a sharpie and rub in a figure-8 motion until all ink is sanded off. This can take a while - and then you have the other pan to do.
Thanks for the comments fellas, use have convinced me . If i'm going to pull 1 I might as well pull the 2 and have a good look .. I have an engine building mob not far in Coffs Harbour who honed my Norton Barrels and checked the valve pressure in the heads also so I can get them to do that part of it if need be .