What you need is to eithrer get onto the Ishop Marine or Boats.net microfiches. open a couple of windows, then:
1) find your engine. Note the part numbers either side of the "join". (you might need to also take the tray into account).
2) for your 1994 powerhead, find the "standard" 200 Hp microfiche, d othe same comparison. if they are coross compatible, chances are the XR powerhead will have a differnt part number, but (for the purposes of this experiment) a leg is a leg.
3) then open a third window. Compare the '94 standard engine with the '99 one. Assuming no major model changes (e.g when the Clamshell turned into something more conventional) you will probably find the same leg. part number. Lower tray may be different to blend in with a different top cowl, but the leg is less likely to have changed. (I have cross decked leg components from a 1997 engine to a 1979 before)
Tip 1: take a careful look at the lower tray - some Mercs had an "exhaust adaptor" which sandwiched in where most engines put the lower tray. I say that as I can't remmeber when the wraparound cowls dissappeared.
Tip 2: It's also worth cross checking the equivalent Mariner / Yam parts lists as some of the part number seems to relate to the paint job....
If nothing seems to cross reference, it's time to get out the measuring stick & start counting splines! Probably doing that anyway at a high level to confirm what you have found.
luck:
