You can get it done on a small budget, it'll be very hard, but doable.
Get a 49cc kit, don't go to gasbikes or any of their people. They are too much trouble to deal with. most of the kits on here are almost the same stuff. Chinese pieces of poopie. It's going to be hard to get a very good very reliable engine kit for under 150. So, you want to buy from somewhere that will stand behind their product, and give you
of mind. I got my kit from Pistonbikes, and they sent it out same day. Call them, talk to them. Tell them what kind of bike you have, and get suggestions from them. If, and only if, you feel comfortable, go ahead and buy.
You will have to instal everything yourself. I am a jet mechanic for the Navy (I used to be stationed in Brunswick), and I still had some trouble. Not like there was anything very bad, just the clutch gave me issues. And so did the rear sprocket and the carb. But, nothing too bad. After that, I had a great time making sure everything was on tight, and I enjoy every second of the ride. Try out Pistonbikes, That's dax and a couple of other places. keep away from anyone you can't talk to directly... get their names, and develop a relationship with them. If anything goes wrong with your build, you can just call them and talk to them, and they will probably go the extra mile to help you out.