I understand monies are an issue , but , a Zen does not come into the same class as a whipper engine .
Average rated HP and RPM : Zen 4 HP 15,000 RPM
Whipper 3/4 HP 9,000 RPM


Thre first and easiest mod .... change the carb over to one like a 257 ( Walbro )
Next ... try out a tuned pipe or two ( some will take one and show performance , while others may not . )
If using the cannister muffler that comes on the engine . Gut the insides out , and double the size of the output for the exhaust to escape ( whether its just a hole or a tube sticking out )

Lighten the engine , by removing the air cooling and converting to water cooled .

If its twin ringed , remove the lower ring to open up the RPMs a bit .

Then , if after this , you want to , and feel confident enough ... you can open the intake and exhaust ports a little . ( intake lower and slightly to the sides . exhaust - high and also , slightly to the sides )

Then ... if your REALLY feeling confident .. you can remove the jug and widen the transfers and smooth out all the uneven and rough edges a bit .

might want to also mess around with different coils , as some are restrictors ( limiters ) for the engines RPMs