Okay, if the timing belt turns out to be good you probably need to do a rebuild. For a good source of information with knowledgeable people, try this site:
Engine Tech & Diagnostics
One of the things I would strongly suggest is using stainless steel exhaust valves from 3 Tech. They aren't any more money that normal ones but last a lot longer. you also might have to go to normal pistons and rings instead of the special ones used in the XFI. The effect on economy should be negligible but the XFI stuff is almost unobtainable today so it's your best choice if you need an overbore.
__________________
No green technology will ever make a substantive environmental impact until it is economically viable for most people to use it. This must be from a reduction in net cost of the new technology, not an increase in the cost of the old technology through taxation
(Note: the car sees 100% city driving and is EPA rated at 37 mpg city)