ST2 heads need water cooling. They don't have deep fins so cooling them with air only could give problems.... You could transplant the cams to the M900 and sell the heads to a unlucky '97 ST2 (and some '98 models in the USA) owner. The first model had head-warp problems and love to leak oil and gunk on the engine. Don't ask me why I know this.... (just bought a second hand '98 ST2 engine)
The W heads are not as dredded as you might believe. The difference between the V's and the W's is highly over rated IMO. My '98 runs neck and neck with my buddy's '95 and I weigh 20 lbs more than he does.