If it cruises ok, & then you barely increase the throttle & it stumbles, then I doubt the problem is fuel pump or filter related.
It is probably a power piston problem. The piston should lift up under load bringing the power step [ rich ] of the metering rods into operation. In other words, there is no enrichment under load. Wrong pri jets or rods can also cause this. Or, did they forget to install the spring under the piston....
Removing the air horn will reveal the piston & should be checked out as above etc, make sure it moves freely up & down.