Help choosing a replacement engine

My Scag 52 inch SWZ needs a new engine. It had a FH500v on it, 17hp, shaft size 1 x 3 5/32. I am considering the following options from Small Engine Warehouse,

FS481V-S26 16hp
http://www.smallenginewarehouse.com/FS481V-S26-S.html

Or

FS600V-S25 18.5hp
http://www.smallenginewarehouse.com/FS600V-S25-S.html

Or
Any other suggestions?

My question is, can I go from a 17 to a 16, or do I need to find a 17 or bigger? (like the 18.5) Also has anyone bought one of these...

Help choosing a replacement engine