actually, pieter went the route i suggested. he dropped the motor out of his m600 and stuffed in a 900!
honestly, i was only 1/2 joking... swapping motors is probably the easiest (and most rewarding) method, but it takes more time/investment. if you REALLY want to go nutz (not that i know anything about absurd projects), try to track down a totalled m900 (you'll want an '02 to keep things "simple"). shouldn't be too difficult. download the parts catalogs for the two bikes from www.ducati.com/bikes/jhtml
and make a comprehensive analysis of what parts are interchangable/compatible.
part out the m900 on eBay or the boards here except for the motor, throttle bodies, ECU, and wiring harness. maybe the swingarm & rear wheel as well. sell the forks, tank, front rim, brakes, etc... to help recoup the cost. drop your 620 out, stuff in the 900 and sell off the 620 once you're good to go!
aside from the obvious benefits, your as far as your insurance company knows, you're still riding a 620
(just don't get caught).