Found this site -
http://www.teedrie.nl/index.html" onclick="window.open(this.href);return false;
It's in Dutch, but you can find engine codes there.
Looks like mine did indeed start as a DF 4 speed.
Tweede is dus de motor die in deze bus zit.
code omschrijving
1 1.6 luchtgekoeld 37kW/50Pk
2 2.0 luchtgekoeld 51kW/70Pk
3 2.0 luchtgekoeld 51kW/70Pk
4 1.6 diesel 37kW/50Pk
5 1.9 watergekoeld 44kW/60Pk
6 1.9 watergekoeld 57kW/78Pk
7 1.9 watergekoeld 60kW/82Pk
8 1.9 watergekoeld 66kW/90Pk
9 1.6 turbodiesel 51kW/70Pk
A 2.1 waterboxer 82kW/112Pk
B 1.7 diesel 42kW/57Pk
C 2.1 watergekoeld 70kW/95Pk
'89 2.1 with Pierburg-esq carb.
ABN/ABM 5sp gearbox. ( I forget which, and now it is under a protective layer! )
“Some people die at 25 years old, and are buried at 75 yrs old” ~ Benjamin Franklin.