People have been replacing the floppy drives with USB emulators for several years now.
However, most, if not all, of the posts on this site were erased so you can't see them...
I googled the following:
psr 740 replacing floppy drive with usbAnd got many hits.
Example: I see a site that sells the item(s) required at:
https://www.floppyemulator.com/yamaha-psr-740-floppy/ Read the responses below the article - in one of them is the part number(s) required from that supplier.
There are several others shown so maybe check them out too - not all emulators are the same - I have heard of a some people having problems with these conversion kits.
Also, there are youtube videos showing what you will have to do to make the change...
Watch one and see if this is something you can do - or, maybe you know a serviceman who could do this?
I did not change my PSR 2100 (still has the floppy), so I don't know much else about this.
Instead I was able to buy a newer keyboard at a great price, which, of course, has the USB drive and many other newer features, which I wanted!
Good luck!