Mower - Swapping Engines - Shaft Issue

I have an older RX model rear mount JD riding mower.  It had a Kawasaki engine on it, the engine is toast after many years of work.  I came across a replacement which is a briggs quantum XM from an old power washer.  The motor should work fine, my only issue is as follows.
 
Issue 1: The old motor shaft was 1" in diameter, the new motor shaft is 7/8", so it's a couple 16ths smaller.
 
Issue 2: The old motor shaft was 3-3/16" inches in length, the new motor shaft is 1.5" in length (a little less than half the length).  The key on the briggs shaft is also narrower than the kawasaki.
 
I need the longer shaft for the pulley stack that drive the wheels and deck.  I'm comfortable with small engines and mower repair and I was debating on just welding the pulleys to the shaft and somehow add some spacers.  I have to imagine though there is a much better solution out there.  Any advice would be greatly appreciated.
 
Thanks,
 
Matt