You could try a little APRS modem from Argent Data or Byonics to connect a GPS to your existing handie.
Back in 2006, I used a Garmin Etrex and Argent OpenTracker with my Alinco DJ580:
and a few months ago I tried a similar system with a simple GPS receiver, OpenTrackerPlus and ADI handie, which tracked me well over Winter Hill.
But recently I’ve been trying out the LoRa APRS system, which works over surprisingly long distances for 100mW on 70cm. This is an independent unit, so it carries on tracking whilst I’m using my Alinco DJG7 for QSOs.