So I looked up this HexaWave HWF1681RA - definitely a MESFET. If it turns out they're shot, you may be able to find NEC brand equivalents. I vaguely recall that I have some amplifiers that have NEC-something-something-8001 or 8002 types that have similar Idss specs. I don't recall the exact part numbers, but you may find a bunch searching NEC/CEL (California Eastern Laboratories - their US distributor way back when).
Ed
