It is very easy to do, but definitely not recommended as the non-turbo motor is not built to handle the heat and stress that the turbo motor will. Swapping in a turbo motor, on the other hand, is even EASIER, and HIGHLY recommended if you must have a turbo.

Just out of curiosity, why do you want the turbo? Is it time for a new motor? Does the original motor run just fine? Most of the power issues I've seen with the non-turbo motors are either normal maintenance related (or lack there of), or simply due to improper transmission detent adjustment. I have bought several of these cars, and the 240's, because the owners were sick of them being extremely boggy, slow monsters. Either the transmission shifted too soon, not allowing the motor to wrap up into higher rpms, or the filters were trashed, and it wasn't getting enough air/fuel. Some simply needed to be driven! When you just tinker around town in an old diesel, and never get it out on the highway and see higher sustained rpm's, you get coked up. Carbon builds in the exhaust, in the manifolds, the head, on the valves, on the glowplugs and injectors.