I run an 18TZ and a 21TM in mine. I like the TZ a little better.
You can make the TZ a drop in motor for less than a 21TM..
http://cgi.ebay.com/OS-18-TZ-P-T-ABC...lenotsupported
You'll need to get these to make it a drop in motor.. Or you can do a little trimming on the crank, and use a OFNA Clutch nut and it will work.
http://cgi.ebay.com/O-S-21912000-OS-...lenotsupported
http://cgi.ebay.com/O-S-21911800-OS-...lenotsupported
http://cgi.ebay.com/O-S-21922000-OS-...lenotsupported
http://cgi.ebay.com/Ths-Racing-Revo-...lenotsupported