From the picture below I doubt you will have enough clearance to put a larger diameter sproket on the axle. Not enough to be practical anyway. At 150 cc motor anyway, I bet it is geared down a LOT anyway. Automatic transmission.... 1st and 2nd with reverse? or just 1st and reverse? Find out the top speed in first and second AND what grade it can climb with two people in it or a full load capacity.
Don't see where you will put the turret on the "back" other than over the axle and using the frame to mount a platform on. Is that what you planned? or a trailer towed? The latter has had poor success in past attempt towing a trailer.
Assuming over the axle, it would almost require sitting on a swivel chair rather than standing. A swivel chair doesn't look like it has enough knee room to clear the back of the driver/passenger seat. Could remove the high back seat and put in a bucket. Still might be kneeing the driver in the back of the head. Would have to measure. If standing up there, it would be top heavy and tippy. Should it occur, I would not want to be the guy standing in the turret... Crunch...
Pain... Screaming... Ruin somebody's day..
Note the roof frame bar in the center going front to back. You would have to cut that out for a midline mounted turret.
I would suggest the turret gunner standing on the passengers seat maybe with the seat removed. Keeps the Center of Gravity Low. Also, eliminates the need to cut the middle roof bar.
MOST of your engagements will be in the forward position. A few on either side and the occasional RPG stepping out onto the road to shoot you in the rear... You would have 360 coverange anyway... Maybe a 3rd port gunner sitting over the engine with gun ports on the sides and back for defense. Side and rear are where the RPGs and Satchel Charges come from. For defense and survivability. The turret gunner can't swing fast enough to defend anyway. The website didn't say what the load carrying capacity is so I would call and find out before you add a 3rd person. You need to know anyway before you load the thing up with a turret, tanks, paint, cannon, etc. etc.
You've got a suspension system front and back which is good for the gunner which it going to get bounced around a lot anyway if standing with that short wheelbase.
Suggest you mount the Air supply to rotate with the turret so you don't have to 'unwind' your hoses periodically. Use the air tanks as a counter balance for the weight of the markers, cannon, and ammo on the front of the turret. The turret's center of gravity needs to be balanced over the pivot point of the turret. Otherwise, the heavy side of the turret will want to rotate to the downhill side on every incline which is MOST of the time. Will wear your gunner out muscleing the turret weight if you don't.
Paintball netting, Super Screen, Pet Screen is the only thing I've heard success with. I use paintball netting. Beg used netting from your local fields.... More netting, less armor.
Then we will proceed with the discussion of turret design.