I pulled up the parts diagram and your 643 radiator cap is on the right side at the top (looking from the rear). Other than that, it does appear to be set up like the 843. You will need to take the grill off to access the cap. That's just a few nuts and bolts. I'd take a look at that cap and see if the seal is dodgy. If you are confident that the leak is somewhere in the core itself, removing that without pulling the engine may be a real challenge. The engine coolant and hydraulic coolant coils are bolted in a mount above the engine cooling shroud. You might be able to unbolt that whole assemble and wiggle it out over the top of the engine but I know in my 843 there's not enough room to do that. Until you'er ready to really dig into things and have the machine out of service for a while, just keep the coolant topped off and keep an eye on your engine temps. I'd go ahead and remove the grill, check the cap and leave the grill off. That way you can actually check your coolant level looking into the radiator. I don't trust those reservoir bottles.