Salbando,
The Monogram OA-4M is a good kit and fits very well. There are aftermarket wheel wells, open air brakes(fuselage and wing), and seats (Aires). I do not know of any cockpit tubs for the two seater. Edward did, or does, make a photo-etched brass cockpit set for a two seat trainer(TA-4 F/J).
I got a template kit by Tri-tool from Sqaurdron as well as a scribing tool. They work very well, but as said before you MUST be patient and take your time. The oval shaped inspection plates on the top of the wing are a real pain to rescribe.
Just think, by the time you finish you Monogram kit Hasagawa will announce the release of an OA-4M, and hopefully the TA4-F/J. I just finished converting a Monogram A-4E/F into an M varient, and it was announced that an M will be out this summer. Go figure! Enjoy the kit. It is a good build and makes a nice model. Find aftermarket decals because the kit decals are worthless. Skyhawks forever. rangerj