I've used Speed springs to replace MCU elastomers in a rear shock with an oil damper. And although a bit underdamped, for the flattish XC in Holland it worked fine.
Does the older Manitou fork have an oil damper? If there is you should be OK if your riding doesn't suffer too much from under-damped suspension. If not and it relies on inherent damping of the elastomers alone, converting to springs means it will have no damping at all.