Lots of weedeaters/weedwackers have clutches.
With a bit of luck you'll find one that you can screw a pocketbike sprocket straight in to.
I have built a couple of cars using weedie motors.
(see attached videos)
I used angle grinder heads for primary gear reduction and sprockets and chain for secondary reduction.
I want to be able to run the car at a nice idle speed and get an acceptable speed, rather than just run the engine at max output all the time, maybe I'm wrong, but I figure that'd be a lot better on the engine
Yep, you're wrong....
Chainsaws and weedie motors work best at full revs, as they're aircooled.
The fins on the flywheel force the air across the cylinder head and keeps it cool.
This is why it's important to keep all the protective casing on the motor
Adding a second axle, or jackshaft is a good idea as you then have the option of playing around with ratios as you go.
It's all good calculating a top speed on paper based on the motor RPM and circumference of the wheels, but once it's on the ground it's a different story.
The weight of the car, the surface you're driving it on and wind resistance etc will slow it down.
I'm not trying to discourage you by any means, but these are all things that I've learned over the past couple of years of trying to build a decent homebrew car.
As I said, be prepared to make modifications and alter a few things.
I don't think anyone has successfully built a large RC car that worked straight away.
Check the sticky thread on top of this forum "List of home made projects"
Plenty of ideas in there.
Check out your local scrapyards and recycling places.
I usually get all my bearings from old angle grinders, fans, powerdrills etc.
Steel or aluminium for framing is usually available cheap if you know where to look.
The only things that really have to cost money with a project like this is the radio components and possibly the wheels.
Now get building!
[youtube]http://www.youtube.com/watch?v=ImT8UW9NZ0w[/youtube]
[youtube]http://www.youtube.com/watch?v=HdC4r5l_-j4[/youtube]
[youtube]http://www.youtube.com/watch?v=qwKJslE_ip4&feature=related[/youtube]