Point No1 What Merlin can be used. Several have said the MkVIII used by the Fulmar MkI but there is something that is confusing me in the wiki article on Merlin variants
List of Rolls-Royce Merlin variants - Wikipedia
Merlin VIII
take off 1,080 hp (805 kW) at 3,000 rpm
but
max power 1,275 hp (951 kW) at 3,000 rpm, +9 psi (62 kPa) boost, sea level with 100 octane
Why would take off power with 100 octane be lower than max power at sea level. I suppose It could be the take off power shown is for 87 octane. It is very tempting to go for the MXII from the Spit MkII which was able to run at +12psi but thats not in production till Sept 39
Point No2 A 4 blade constant speed prop is obviously too early but the Blackburn Skua had a 3 blade DeHavilland prop which was either variable 2 pitch or Constant Speed I cant find out for definite. There were plenty of variable pitch props going to the bombers so maybe the Navy can squeeze a couple of hundred out of the Ministry.
Point 3 I have been shot down in flames for asking for 12 x .303 but I wonder if there was room to increase the ammo capacity a bit maybe up to 20 seconds of firing time even if it was only for say 4 of the guns.