First, you'll need a dremmel tool with a cutting wheel.
Second, you'll need a ziplock baggie.
You will have to measure the shaft prior to cutting. A good ruler to use is the shaft from your previous engine. Basically, remove the clutch bell, fly wheel and clutch nut from the old engine. The length of the shaft from engine housing to tip should be the same for the old engine (.15) to the new engine (.18). You can verify this by mounting the new engine on the engine mounts and holding the .15 next to it to make certain that you are not shorting the shaft to much.
Once you are certain you have the right length, mark it with a sharp china marker or some other marking device. Place the engine in the ziplock baggie and poke the shaft to be cut through the side of the baggy. Placing the engine in the baggy is for preventing filings from the shaft from entering the carb. Grab the dremmel tool and cut away.
Take your time on the cutting. Just remember you can always cut more off, but you can't put more back on. If in doubt, cut less off than measured. Put the clutch nut, flywheel and clutch bell on the engine and fit it to the chassis. If too long, cut more off.
