What I did before my upgrade from Tillotson to the WMC (Walbro) was to compare the parts listings for the fuel section of the engines (1980 vintage to I think 1985) via on-line vendors. From there I learned what parts were needed. IIRC, I got a used flange behind the carb and ahead of the reeds and a used reed body from a WMC-carbed motor. I think I had like $250 invested in parts total. The biggest pain is the grinding of the cowl "hook" to allow the carb to fit. The rest of it is bolt-in.

Now, if/when you do go the WMC route, you will need the plastic pieces on the front of the carb (not the inlet silencer as it won't fit under the cowl) that allow you to prime the carb when cold, etc. One can find these pieces on the Bay of E, but it might behoove you to purchase them new as what is typically sold there is worn. I did not opt to get the piece that guides the primer knob on the motor cover and mine just protrudes from the same hole that the Tillotson knob does.

This is what I think you have:
http://www.marineengine.com/parts/mercur...block-and-cover

This is what I think you need:
http://www.marineengine.com/parts/mercur...-serial-group-1

Notice the carb mounting flange is different.

The pieces in front of the carb inlet as found here are what I used:
http://www.marineengine.com/parts/mercur...a/carburetor-25

Good luck, keep that motor running!

I should probably post pictures and this might give me an excuse to learn how to do this.