Saab 340 pilot reported the right engine torque decreased during initial climb in IMC causing aircraft yaw. The flight crew reset power and requested a return to departure airport. During vectors for the approach; both navigation radios failed and the crew was not able to intercept the approach course. Flight crew diverted to an airport with visual meteorological conditions and landed.

Narrative

On Day; I was the Second-In-Command on a Part-91 Ferry operation flying a SF-340 from ZZZ to ZZZZ operating on a special flight permit. The aircraft was released for flight by Company X. On the flight; I was the pilot flying and the Pilot-In-Command was the pilot monitoring. METAR was (W) reporting 140/08 10SM; light rain; 700 OVC. I always include in my takeoff briefing should I notice an irregularity; I would call it and let the PIC make the decision for abort or continue. During takeoff at approximately V1; the right engine Torque (TQ) increased to 108%. (Maximum power in the Saab 340B with CTOT-engaged is 100% TQ. CTOT is a device on the aircraft to set desired Torque for takeoff and go-around; Constant Torque on Takeoff. When this occurred I called Right Torque 108%; the Captain said to continue the takeoff and take the aircraft airborne. As soon as we were airborne; the right engine Torque decreased to 40% TQ causing a right yawing moment. (In the Saab 340; if the power lever is pulled back with the CTOT engaged the torque will decrease to approximately 40%). I pushed both power levers forward and grabbed the yoke to maintain directional control. The PIC requested to return to the field and ZZZ Tower gave us left traffic pattern. We leveled out at 3000 ft MSL (around 1500 ft AGL) in IMC and turned the CTOT off. Once the CTOT was switched off the TQ on right engine returned to its previous power setting of 80% TQ (making us believe there was a malfunction with the CTOT). Both Power Levers were then reduced to around 60% TQ. The PIC requested vectors back to land since we were in IMC.We were unable to fly an RNAV approach as the GPS database did not have approaches installed. The PIC requested vectors for the ILS Runway XX in ZZZ. While on vectors to intersect the localizer; both NAV 1 and NAV 2 radios failed and were unable to capture the localizer; glide slope; or VOR/DME. I verified the frequency was correct and approach mode was on. Tower asked our intentions since we overshot the approach course. The PIC requested vectors to an airport with VFR weather. We were given a heading (west) to avoid weather and were transferred to ZZZ Center however; we weren't able to make contact. I tuned in to ZZZ VOR and ZZZ1 VOR to trouble shoot the Nav radios. We received conflicting readings with DME and radials so we confirmed it was a NAV failure and not a station failure. Another aircraft on the same frequency was able to relay messages back and forth for us to ZZZ Center. After climbing to 4;000 ft and out of weather; we were able to speak with Center. The Captain had comms and informed ATC of engine malfunction and lost navigation. Center informed us they had [given us priority] and vectored us south to ZZZ2. I descended the aircraft to 2000 ft to get below the cloud layer and executed a visual approach to Runway XY. I landed the aircraft in ZZZ2 with no further incident.