Money on top end airleak.
Polini 70cc head right - not the 50cc one?
it has a decompression valve? check that it actually seals / isn't stuck open or failing / that it is installed properly with appropriate sealing washers, etc.
head/base gaskets intact and actually sealing?
do a leakdown test
do a compression test (which it sounds like you did i.e. turning it over by hand.)
spray the whole thing down with soapy water and get it spinning and watch for bubbles.