i would say i nice 572ci 700 to750 hp motor would be a good choice to help keep the xr alive or you could go with a 598 or 600 ci same hp rating but more TQ,
if you take it easy getting on plane and keep of the stick when getting air either motor's will be fine on a xr for a while .
i had an xr on a 2005 28 pantera with a 600 ci 750 hp motor and the drive lived for over 80 hour's until i sold the boat and from what i herd the new owner put another 50 hour's on it before a rebuild , so it all depend's on how you are going to drive . iv herd of guy's blowing xr drive's with 525's but they do whole shot's from a dead stop
lot of nice looking motor's in the oso classified's