Using a MOSFET on these single transistor Pleiades powwer amplifiers?

At your risk. Please take all safety precautions. It looks tricky as by looking at some datasheets the on channel resistance is a fraction for an ohm. Typical current is a few amps. So it seems a step up output transformer is needed with a step up input transformer. Another option since depletion FETS do not need a say +4V at gate to turn on, is to use JFETS such as 2N3819. Many in parallel.