The spring return is a good safety set-up. You can get the right kind of spring from DuraTrax. I got mine through Tower Hobby under the parts section for a Max ST, DuraTrax part # DTXC 9647 (http://www2.towerhobbies.com/cgi-bin...?&I=LXVG87&P=K), around 3 bucks.
If you don't have a place to connect the spring to the motor, use a small zip tie around the base of the engine head & connect the spring to that. That's the set-up I used & it's worked well for me.