VW I-4's are cheap and plentiful and get good mpg.  If you are happy
with it, you can keep it running forever off used engines for next to
nothing. VW I-4's and Subarus can be done, in your garage, for similar
money. Subarus take a bit more labor, but you get a better fit, and a bit
more torque (EJ22).
    It sounds like you bought a van with a good I-4 conversion.  If you are
happy with it you will have a frugal, easily maintained, and cheaply
repairable van for a long time.  I would stick with what you have, and that
is from someone who does Subaru conversions.

    John
    East Coast Vanagon.