Hi John
Ditching the SLA battery and changing the motor, prop and speed controller is a big upgrade !!
So getting the right info will save you money.
The original Perkasa's would do 50 knots plus !!!
I have two 1/35 scale Perkasa's both with 37mm 3 blade props, 60 Amp ESC"s, 5800 mah Lipo batteries.
One has a 35mm brushless motor 3639-1100 Kv and the other a D3548/4-1100Kv.
Both get up on the plane and perform over scale speed.
Good run time of over 1/2 an hour.
Turning is not great as they like a long turning circle, but, can be help by raking the rudder forward by 6 to 8 degrees.
The balance point forward and aft is very important ,but, with access hopefully to under the forward gun you can move stuff around to get it right.
As the battery is the main weight problem.
As for moving guns you can use small servos, but, I am it to that.
Harry Smith