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  I started out with a 49cc youth-ATV (aka. Pocket-Quad) - a Wildfire-Motors model QF49E,
bought on eBay new for $380 with shipping. I chose this one over the others because it had:
A) Full suspension - Independent front wishbone, rear Swingarm             
B) "keep-it-simple" Chain-drive, Solid rear-axle, Disk-brake                   
C) Electric-start - and bonus, a 12V motorcycle battery already built-in
D) Replacement parts available from the dealer                                        
E) Cool knobby tires already on it                                                                

  Out of the crate... to put it mildly, a typical piece of Chinese-Crap. Frame twisted, parts falling off,
every bolt loose, plastic cracked, no instructions, etc, etc. I still laugh when I look at the welds. Of course
I gassed her up and took it for a spin. Not much of a thrill for an adult, maybe 30 mph on pavement.
Off-road it's nothing but a death trap. Brakes non-existant, chain falling off, clutch smoking.
Really glad I didn't buy this for a kid to ride !

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So off to the workshop it goes... I stripped all the parts off and cut off the top-half of the frame,
foot-rests, brackets, plastic, etc. I don't have a welder so I just soldered-up some 1/2" copper
plumbing-pipe and fittings to build the roll cage - it's very strong!  Gave it a couple coats of spray-paint
and bingo. Had to fiddle to get the motor/chain/sprocket all aligned and tensioned properly. Then
re-assembled this time with some grease and loctite. Had a small sheet of aluminum diamond-plate that
I used to make the servo mounts and "hood". The lexan windshield doubles as the radio-box cover.
I put in some lighter springs (no rider) - and discovered the shocks were fake, no oil or air - bummer.

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For 50 lbs, need lots of steering power, I thought about ganging-up some smaller servos, or rigging up
a windshield-wiper motor, but eventually I bit the bullet and bought the Tonegawa PS-050 Servo ($190).
Puts out a whopping 1800oz-in when supplied with 12V, has aluminum case, warranty, etc... a no-brainer.
I'm using the existing steering-mount and direct-driving it (no linkage), with a steel Spider-coupling.
The rubber insert functions like a servo-saver. For the throttle/brake servo I used a Futaba S3306
1/5 scale - 333oz-in (used for steering HPI-Baja). The throttle doesn't need that much, but I will want
it to pull hard on the brake. It was fairly easy to shorten/mount the existing cables, then link them
to the servo-horn. I've also added a fail-safe so it will throttle-down if the radio signal is lost.
The radio system is a Futaba 3PM FM-75Mhz with the 3rd channel operating a Kill-Switch relay.

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All in all it works pretty good. Plenty of power and speed, steers like a champ, stops on a dime.
After several hours of abuse, nothing broken or bent yet. Just a lot of blue paint scraped off.
Total cost around $900 - less than a Baja or FG

The ride is very bouncey which makes it hard to keep straight - Version two will include a functional
suspension with real shocks. Probably ditch the electric start to save weight, and powdercoat the frame.
Here's some links to similar projects, E-mail if you have more and I'll add to the list:
www.tonegawaseiko.co.jp
www.futaba-rc.com/radios/futk15.html
www.youtube.com/watch?v=7D0MwxEkikg
www.youtube.com/watch?v=SuTBbCTpV2E
www.youtube.com/watch?v=vWoujoIWYiY
www.youtube.com/watch?v=6phNcx1744s
www.rcuniverse.com/forum/m_7654560
www.rcuniverse.com/forum/m_4957027