I'd use a pinion gear puller like this http://cgi.ebay.com/GWS-MINI-MICRO-P...item3ef67efbeb to safely remove pinions. They should have them at your LHS too.

To remove and replace a stock Tamiya pinion, I use my gear puller to remove it, and then simply press the pinion back on the new motor. I start the pinion on the shaft by hand, and then once it is started, tap it with a hammer while holding the motor. Take care not to bang too aggressively with the hammer, or you'll ruin the bearings. Lots of little taps... I also don't put the motor up against anything solid when I do it, hold it with your off hand.

They'll be people who say don't do that...but I've been doing it that way for many years now with good results. If you're paranoid about breaking it, put the motor in the freezer overnight (that will shrink the shaft) and heat the pinion with a micro-torch (that will make the hole larger) to make it fit together better. Green Loc-tite works wonders if the gear doesn't fit too snugly. I'll have my gear puller at battleday, and I can show you how I do it in about 3 minutes if you'd like.