From what I have read, yes you can use the newer fans in place of the olders, although I have never done the switch myself. It sounds as if your fan clutch may not be working. It should be fully engaged at a coolant temperature of around 95 degrees.
In a pinch, hotwire the aux fan to run all the time or put a switch by which you can turn it on and off.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13