G'day Mate,
I bet it is running lean as tank level drops, & you can't get enough fuel up to it.
Lower the tank, centre line must be even with or no more than 1/4 to 3/8 below the needle valve, then tune it properly, use this web page,
http://saito-engines.info/throttle_setup.html
Then you won't need pumps or regulators, or anything else to make it run sweet. Not even on board glow.
Tank height is the number one problem with inverted engines, of any size.
Get that right & the motor will run right, get it wrong & you know what happens, because it is happening to you now.