ok here are various webpages that may or may not help you...
of interest is the first one that lists how many guns each variant carried. That Martlet I which you would be modeling had 4 guns instead of the 6 that the Tamiya kit comes with, I believe the outer gun was removed but let me check on this. A little putty will fill that extra gun right in!
http://www.vectorsite.net/avwcat.html
another noting the differences between marks:
http://fleetairarmsig.netfirms.com/articles/martlet/scottspencer-martlet/martlet.html
PM me your email and I have a good reference in .pdf I can send ya... it gives a lot of info on the Martlets.. it looks like for the Mk. I you'll have to sand off that upper scoop and cut off and sand the inner scoops. The cowl flap panel lines will need to be filled and sanded, then right in the middle of the flap scribe a new panel line.
OR - you can do what I did and throw caution to the wind on that 804 Sqn one and do it OOB... the Lybian one I went after correcting the cowl but the first one I was blissfully ignorant and "did it my way"...
-=edit=-
something that bugged the bejeevers out of me on the first one was "what color was the lower wheel carraige?" It is semi-gloss black, just like the Wildcat... also the interior is a softer green than Zinc Chromate Green, you should be able to get away with lightening the green zinc with a few drops of white... I guess it would be more of a "sage" color...