You will probably have to decide on a cowl diameter by measuring the engine and making paper cowls until you decide what looks best. A cowl source is http://fiberglassspecialtiesinc.com and they have a ‘radial cowl’ section with a bunch of different sizes.

Williams brothers is one source for the gun http://www3.towerhobbies.com/cgi-bin/wti0093p?&C=QHF
and here are some pilots http://www3.towerhobbies.com/cgi-bin/wti0093p?&C=QHA