The opposed four cylinder push rod aluminum engine part is just fine.

with push rods, no timing belt to replace, wear out, adjust or break.
Very nice that way.

all the engine needs is ........4 valves per cylinder,
and a closed deck block,
and flat bottomed heads, with 'real' head gaskets.
and better breathing, like in intake runners, throttle body ec.
that would be just perfect.

btw...............individual exhust ports, which the waterboxer has,
....one header pipe per cylinder is nice too..
very handy for troubleshooting.  So keep that feature.
( you see one header pipe running 100 degrees F cooler than the others ..
you know something's not right in that cylinder.
Takes 2 seconds with an infrared temp gun.
very nice design in that regard ....idividual header pipes for each
cylinder. . )

that's what I'd like to see ...
2.2 to 2.4 liters, push rods. flat bototm heads, better breathing and 4
valves per cyl.
that would do it just perfectly.
more evolved fuel injection would be nice too ....sequential and 'lost
spark' igntio with no distributor is very nice too.

( sounds just like a push rod EJ2 subaru engine ! ..if there was such a
thing ..)

oh ...and on any engine ...water pump driven by timing belt ..
 is dumb !....since water pump wearing out can blow up your engine when the
timing belt jumps time, if it's an interference engine ..
another nice thing about the waterboxer...
the water pump shaft getting wobbly, by itself, won't blow up the engine.
Very smart design that way.