Welcome to the site! What part of the world are you in?
39 mpg is pretty unusual, must be a 4-speed 2wd.
Good tailgates are hard to find. Any '81-'82 LUV or '81-'87 Isuzu P'up will fit. The '86 P'up didn't have "ISUZU" stamped into the sheet metal, they had a decal instead. If you can find one of those, your truck won't be sending "mixed messages".
First priorities:
Replace the timing belt, it's an interference engine.
Replace the transmission oil. It uses motor oil, and heavy gear lube will kill it.
Jack
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'83 Isuzu P'up Diesel LS - Original Owner
JoeIsuzu@IsuzuPup.comIsuzuPup.com