The platform evolution was Fortune I, Fortune II, PE, PE+, Game King.
Bill validation support was introduced at the PE+ era.
Early PE+ platforms did not have bill validators and many PE+'s were converted PE systems.
For these systems there was side-car add on. This was a big ugly box bolted onto the right side of the PE+ machine.
They were not popular with the casinos as this meant that the machines were more spread out. Some casinos built their own boxes and mounted them on the top of the PE+ and also integrated player tracking systems. This allowed for the higher density placement.
Getting back to your Fortune II question. Officially there are no supported Bill Validation functions.
There are 3rd party bill validators that use PCM - Pulse Code Modulation that can simulate coin inputs on almost any system.
In short you program that a dollar is worth 4 coins and it spoofs the coin mech into thinking 4 coins have been entered.
These are very popular for use with PinBall Machines and other legacy Video games that were never designed for Bills.
ICT makes a variety of them -
https://www.ebay.com/bhp/ict-bill-acceptorYou would never have seen a Casino use these as they would not have existed when the Fortune II's were in casinos, nor would the Nevada Gaming Board have accepted these as the accounting regulations require tighter integration to the slot accounting system.
It however can be done. For me the question of making appealing aesthetics (especially for home use) would be more paramount.
Those PE+ sidecars were ugly.
Personally I would be more inclined to go down the road of getting a bill breaker. You insert your bill and coins or tokens come out.