I don't know where you would find a larger snorkel to fit? you could hunt around some wreckers at other model cars, but I doubt you will find a model with a larger one.
I was looking at this for a 31 and ended up just using a home made enclosed pod filter with large 4" flexible ducting into the enclosure from underneath the front.
save up and buy a better performing vehicle alot lot hassle, or sell this 31 and buy another that has work already done on it...
unless you spend some decent money on it the gains will be minimal... and doing the manual conversion aint that cheap either, finding a gearbox under 600 will be hard then you need the other bits, ie. drive shaft, clutch pedal box, cross member all are wanted bits plus manual ecu possibly...