I think you only need liability insurance and the registration is still only $5 so whatever. Honestly, the ticket is something like $15 for not having your scooter registered so I figured I'd take my chances. It's freaking ridiculous and I've never had a problem. Hopefully they have better things to do. I've heard of them cracking down in places like FC where people are riding in traffic but I figured on Trough Road I'd take my chances. haha
back to the topic, I got to the same place sick of paying $35-60 everytime for shuttle since we go by ourselves a lot. I picked up a cheap mtn bike on Craiglist and got the kit. 3 hours or so had my motor-bike. Honestly, this thing is fun just for the sake of it! I ride it around the neighborhood all the time just for fun. For a couple hundred bucks all in I made up my money on it pretty fast.
For shuttle it is great because it is light and portable. You still have to transport it upright or the gas leaks so I added a bike rack to my raft trailer. A trail bike would be great if you wanted to deal with full on registration and were able to transport one on your trailer. Even with the new laws not having to deal with full license plating would be worth it, and having a motorcycle license, etc. On paved, flat, or somewhat uphill it cranks pretty well. If the hill starts to get too steep, the little engine will bog and you just have to pedal a bit to help it out. Usually just easy peddling in a high gear will keep you going. And we're talking about a 220 lb guy here. So it works really well under most normal conditions. I know a couple hills where I have to help it but it's more of a keep your feet moving help, not a working hard help, and those are long steep hills.
Downsides are it does vibrate quite a bit and those kits are chinese made so don't expect the greatest quality. I broke a couple parts putting it together but nothing that I couldn't work around. Cheap cast metal and stuff like that. I've clocked mine going almost 30 on flat pavement, but you can change the gearing to get faster or more climbing power too. Some kits have a gearing kit that changes it so you can use your motor through the bikes gears, the kit I have is essentially one speed out of the motor. it's a blast though. does take some maintenance and tweaking so knowing basic motors is good.
I just lock it up at the put in or take out, whichever works best and scoop it up when needed. Usually starts up a couple conversations. I've mainly used mine on the ark and colorado. never even been questioned.
as far as using your GT, the frame has to meet some measurements, you should be able to find them online. depending on what kind of kit you go with. here's mine.