The two stroke helps lubricate the pump and makes the engine smoother.

Also was having a think last night.
Didn't Sierras come with a Peugeot 2.3D engine?
Now if you could get the gearbox off one of them then Peugeots XUD9TE engine should bolt straight upto it.
An XUD9TE with a well modified pump, straight through exhaust and an FMIC will make 140-150bhp.
If you can get an XUD9TE with a bosch pump then you can run it on 100% Vegtable oil too.

Food for thought?

Get it... Vegtable oil / Food for thought

Ahh my comedy genius is wasted on you lot