You could take a half pound bird, (feathers included), feed him a half pound of lead, spin him around in your arms so the centrifigul (sp) force pushes the lead to the approximate correct CG, allow him to regain his balance as he's probably a bit dizzy from the spinning and then let him go. Not sure where the servos go.
Flying lead has already been done..... its called the OS40LA engine, weighs about the same and lead BUT the lead has slightly better power.
I LIKE my OSLA40[>:]...it's proven to be very reliable.