Rincon Cylinder Block Swap

I have a 2005 Rincon 650 with a Highlifter 715 big bore / cam kit installed. The machine has a total of 2400 miles on it. I've only ridden 200 miles since the big bore kit installation. Recently use of coolant increased. Simple tests and observations revealed it was coming from the leaking head gasket into the cylinder. Subsequent teardown this week revealed much more. When the big bore kit was installed it reguired overboring the original cylinder and pressing in the new overbored liner sleeve. The leak occurred when the sleeve liner slid down inside the original cylinder allowing the headgasket seal to be compromised. This distance was only a few thousandnt's of an inch, however, any is too much. There may be a lip which had to be precisely machined for the liner to stop against in the original bore but I don't remember one. Further investigation into this will reveal that answer.
QUESTION ONE: Can I install a new 680 cc cylinder/piston with appropriate gaskets? Or do I go back with the original 650cc stock setup? Oddly enough the 680 cylinder is $160 cheaper than the 650 cylinder. All other parts are the same or very comparible.
The other issue which almost makes me glad this occurred is that the Highlifter cam is trash. The supplied high lift cam is a stock exchange cam which has the face of the cam welded up and them remachined and polished. Upon teardown we found the facing almost entirely gone with the lifters trashed and the cam lobes looking pretty much as they were when they were made, lumps of weld facing. Only 200 miles, over a period of three riding seasons, easy breakin with a 55 year old riding the woods of Kentucky with a couple of fresh Honda oil changes of the appropriate quality and quantity and a motor about to self destruct. Now I wonder about the crank and rod bearings and everything else that turns inside the cases. A couple of good flushes at least them maybe another upon assembly. I'm glad I didn't turn in my old cam for the core fee. At least I have that to rebuild with.
I just found this out Wednesday and will contact Highlifter Monday the 29th. to inform them of their quality control issues and what remedies they may consider.
If my machine shop failed to properly machine the cylinder then that's my baby. But the cam failure is theirs and they need to know this.
So, will a new 680cc cylinder block work?