I believe they do make OVAL Chimney liners, that would probably fit yours.
Yes, you can put in a power vent, http://www.Fieldcontrols.com
you can see how they are installed, they have manuals online.http://www.fieldcontrols.com/venting.php
ON some keystoker models they offer a DIRECT VENT, which mounts directly to the stove, and exhausts out a 4" pipe to the outside.
A POWER VENT mounts it to the outside, and running the pipe to the stove and power to the powervent, you also need a flue switch on the stove, in case the venter fails it will shut the stove down. Probably run you $400-500+ depending.
Do a search on Power Vent on here, to get some ideas.