Sourcing a second hand motor for your Rorty buggy should be a fairly straightforward task, but there can be a few pit falls for the unwary. The object of this guide is to ensure you end up with all the necessary, and correct parts.
The simplest method of obtaining a second hand motor for your buggy is to buy a complete running bike. That way, you're assured of having everything you need. The downside to this is the large initial outlay of cash, although any surplus parts can be sold to recoup some of the outlay. EBay is a good resource for selling unwanted items to a large market.
Not everyone is in a position to purchase a complete bike, and some couldn't be bothered breaking one up, and advertising the remains. So the second most reliable means of securing your motor, is to buy it from a motorcycle wrecker, along with all the necessary components to get the motor running.
Some motorcycle wreckers are fairly switched on, and if you tell them what you are intending to do with the motor, they will sell you a "car kit", which should include all the stuff you need.
Here's a quick checklist to make sure you come home with everything you require:
 

Engine/Gearbox Unit

A bit obvious you might say, but make sure parts haven't been poached off it. Check all the engine covers are present, also the sprocket, starter motor, alternator, gear lever, clutch slave cylinder (if hydraulic), all sensors, neutral switch and carburettors or injection system. Ask if they have removed, swapped or replaced any part of the engine package, especially the throttle bodies. Take the engine mounting bolts too, as the rear ones especially, are usually an odd length, and aren't easy to replace. Ask if any history is known about the donor bike. Check around the engine block for any signs of damage.
Most importantly of all, make sure the engine actually has an un-altered engine number!

Air box

It will probably have been removed to make it easier to strip the bike, so be sure to get the whole thing, so it will fit straight back onto the throttle bodies. You'll most likely end up throwing the main part of the air box away, but until you decide how much of it you need to convert to a larger free flowing filter, just take it, along with any hoses, clamps etc. that belong to it.

Exhaust

You'll probably only need the header flanges and clamps, as the rest of the exhaust will need to be fabricated to suit the chassis, although some "Y" pieces can be handy. Certainly don't waste your money on a sparkly system, as the wrecker will want good money for it. For all the bits you need off it, it would be cheaper to ask for a damaged system off a dropped bike, as they're normally thrown out. The same goes for the end can. Bike riders are always looking for good condition cans for their smart street bikes. A slightly damaged one will work just as well, cost much less, and the bad area can usually be turned towards the chassis..... out of sight.

Radiator

The original radiator off the bike may or may not be what you want. If it is a split system, in other words, two separate radiators, then it is probably too much of a nightmare. If there's a separate swirl bottle/filler neck, make sure it's complete. If it's part of the frame, ask if you can have a stand alone one from another model. Make sure the temperature sensor is present, and doesn't have its end broken off.
 

Oil Cooler

Take the original one, or even better, ask if there is a larger one from another model. You may end up installing custom oil lines, but take all the original lines and fittings anyway.
 

Wiring and Ignition

With the exception of the driving lights, indicators, horn and speedometer, take the lot, no arguments. You may be told you don't need this bit or that bit, but until you have the motor installed in your buggy, and running, just have the whole thing. The required electrical system should include all temperature/pressure/neutral sensors, loom, rectifier, fuses, solenoid/main fuse, CDI, coils, relays, fuel pump, switches, and tachometer.