you could put mechanical gear in but you're on your own in figuring out where to put the servo and how to route the pushrods. Mechanical gear aren't really meant for larger, heavier, scale planes. The servos will strain to pick up larger wheels and you can forget about scale struts. Should a retract servo stall for some reason you could end up with a pile of sticks as it can quickly drain your battery. The Top Flite kits are all engineered for pneumatic gear so most of the hard work is already done for you. Don't be put off by the air set up. It's easier than it may seem initially. Ask questions here and you'll have all the help you need!

Mark