25hp motors have different engine tin than the 36hp.  Out of the car, you d
see a different kind of engone head bolt- uses an allen wrench.   Differnt
heads.   Have head of 25hp heads bored out to take 36hp ps and cs for that
extry oomph.  same case,  check the serial number.
  
25hp motors should have a wider fan belt

also 36hp nut on gen pulley (which looks wierd.  )  Howver 54,55 maybe 56 id
have to look it up also use 36mm nut on pulley.   Also used was an oil filler
that one does not unscrew to cap from.  instead, one pulls off a spring tab,
and pulls out the oil filler from the case/gen stand.  Actually, the 25hp
used a 'acorn' shaped oil filler (same part number), norrower at one end, not
one with the same dimensions as the later one with screw cap (in case anyone
has an acorn oil filler kicking around in a box of greasy stuff, i would like
it please) (and a tab, which is held by the gen band bolt)  These 25hp/early
36hp didnot have a place for to take the screw that holds down the oil filler
w/screwcap.

The vintage/July issue og hot vws a couple years ago had some good shots of
beautifully restored type 1s with correct 25hp motors.

also, distribotr cap is flat, cords come off from the side, not the top.

if you already know this stuff,  you spend entirely too much time on your
car.  Whats your chassis number?