I am an Ercoupe owner and pilot. Do you plan to use the plane for your missionary work? It is a bit small for that from the descriptions a friend from Andrews University, who regularly flies missions in Bolivia, has laid out for me . If it is for personal use, you have a great airplane in your sights.
I understand your desire to restore one. If you could get an airworthy plane for $20,000, expect to put at least $10,000 more into it with the modifications you would like. With the big NACA airfoil on the plane, I do not believe you will get much more speed out of it even with your planned engine switch and perhaps a prop optimized for the speed you seek.
If you have already found the actual Ercoupe you want, my suggestion would be to an 0-200 crank in the engine (not sure what the plane will have, and that should bring you to 100 hp). Make sure the center section of the airplane is sound by having the rivets ultrasonically inspected. There is a Service Bulletin out for how to do this... Make sure all the AD's are taken care of and depending upon the prop pitch, have it modified for cruise or replace it if it cannot be modified.
I would leave the metal wings alone or find a C, D or E model without rudder pedals that still has the original fabric wings.
There is a great type club for the airplane, the Ercoupe Owners Club and a separate Yahoo Group dedicated to technical aspects of the Ercoupe. There you will find very knowledgable people who will gladly advise you on taking an orginal Ercoupe and making it a great classic without trying to convert it to an Experimental or Exhibition airplane. Plan to invest some money to do some of these things. This too can cost you upwards of $10,000 above the base price of the plane you find.