Look in the Kawasaki parts manuals for the differences in parts between an FS and FX engine of the same model series. Find the price and availability of these parts. Start with the intake manifold, gaskets, carburetor, inlet adapter, inlet air hose, then move on to the filter support brackets and where it attaches to the blower housing and other small parts.
Let me know what you find.
The addition of a Donaldson two stage air filter assembly reduces the ability of a naturally aspirated engine to breath. The intake manifold and carburetor on an FX are changed to compensate for this in order to maintain the correct air/fuel ratio for proper engine performance and emissions compliance. Along with this comes a reduction in rated HP, not a big deal, just a fact.
A Donaldson two stage air filter system can be installed on some older engines with no ill affect to the engine performance such as running rich in fuel. As emission requirements have become more demanding since 2010, engine manufacturers have had to make changes in order to continue using the Donaldson two stage or canister type air filtration system. You will become aware of these changes as you research the parts manuals. The changes in emissions are the reason for Kawasaki to drop older non compliant engine lines and introduce the FR, FS and FX engine lines.
As I have already mentioned, the cost of the parts to change an FS engine to an FX engine approach the cost of a new FX engine and this is why my customer had me repower his machine with a new FX engine.
This topic has been discussed to a great extent on another forum. In theory it sounds like a logical thing to do, however, technically and legally relative to engine warranty, it is not.