What you are seeking is SCREENMULT which is define by platform as '1' or '2' in Master.h.
If define at 1, it will be : 320x240
if define at 2, it will be : 640x480
You may use the 'raspi1' makefile which only compile the picosynth, picodrum, pbsynth, opl2 synthengine. I think other engine will be too fpu for the pocketchip.
And last thing, try removing the -DDEBUGPRINTF from the makefile, it will be useless.
If it works fine tell me and take one photo, I think this hardware is cool and it may handle midi
Johann