Modern 750cc motorcycle motors make 150BHP.  Not suited for vanagons.

You own a 4500+lb brick.  It is a "light truck".  I suggest picking a
powerplant that matches the task.

The real holy grail is a broad torque curve.  Say >100lbs-ft from 1500 to
6500 rpm, good fuel consumption, long life,...

Horspower is a derived number that has little meaning in application.
Torque turns the wheels.  So...thewestyman has 1.9TDDs that supply gobs of
torque but really don't have a high horsepower rating.  They are very
suitable for powering a light truck shaped like a brick.

A peaky motor is not the answer.  Turbocharging is also a suitable solution
for gasoline motors of small displacement.  However it still boils down to
available torque over a broad range of motor speed.