Major modifications:

Done by several companies, these are listed below along with the major conversions they did.

Turbotech Magnum:

300hp Lycoming TIO-540-J2BD, 5500 pound gross weight, 152 USG fuel capacity,  service ceiling 24000'. STC currently owned by Seaplaneworks

McKinnon Enterprises / McKinnon-Hickman :
260 - 270hp Lycoming engine conversions, 5500 pound gross weight Land and Sea with Strengthening of wing structure, Modification of hull compartments for buoyancy, and strengthening of landing gear, 4700 pound weight limit without those modifications.

Metalized wings and flaps, retractable floats,  metalized outer wings and flaps. STC currently owned by Seaplaneworks

J. Ray McDermott :
240 - 260hp Continental engine conversions, horizontal stabilizer modification, 5400 pound gross weight increase, Cessna dual landing lights in each wing.

Dean Franklin Enterprises / Gander Aircraft :

240 - 300hp Continental engine conversions, 5400 pound gross weight increase.

Lee Mansdorf / Pacific Aircraft Engineering Corp. (PACE) :

300hp Lycoming R-680 radial engine conversions, 5500 pound gross weight increase,  one piece windshield, larger two part cabin entry door, 27 gallon extended range outer wing fuel tanks.

Link Aeronautical Corp. / Lockheed Aircraft Services :

260 - 295hp Lycoming engine conversions, 5500 pound gross weight increase,  22 gallon extended range fuel tank in each upper engine nacelle.

TEAL or New Zealand Tourist Air Transport conversion :
A modified McDermott conversion using the 260hp Continental engines and a modified Franklin Gander upper nacelle.

200hp Ranger original factory engine, 6 cylinder inverted inline.

240hp Salmson inverted V-8.

240-260hp Continental O-470 or IO-470 6 cylinder horizontally opposed.

300hp Continental IO-520 6 cylinder horizontally opposed.

260hp Lycoming GO-435 geared 6 cylinder horizontally opposed.

270-295hp Lycoming GO-480 geared 6 cylinder horizontally opposed.

300hp Lycoming TIO-540 turbo-charged 6 cylinder horizontally opposed.

300hp Lycoming R-680 radial.

Modifications done on the Widgeon have been extensive and wide ranging

The most common are engine conversions, retractable tip floats, one piece windshields and metalized outer wings and flaps.

Covered below are the primary engine conversions, other modifications will be taken up at a later date.

Engine types and conversions are listed below.

Grumman Widgeon Owners Group

                     Fuel Tank Modifications:

Standand fuel tanks: 108 USG total

Pacific Aircraft Eng Corp: 154 USG total (1 tank added to each outer wing panel, connected to main tanks.

McKinnon: 176 USG total (similar to PAEC modification with larger tanks.

Gander: 50 USG (25 USG added to each wing float)