It can be done. I can get pictures if you want. Mine is recoil/electric both. The recoil starter is all plastic-but tough. It has a place on the front left side that goes around the electric starter gear, so you don't have to notch it or anything. Remove the old flywheel cover, put the old LED into the new front cover and bolt it all on.
You need not only the starter itself, but also the rope handle and plate, rope guide and bushing, pulley that bolts to the flywheel, and all those little pieces that work the start-in-gear protection.
If you bought all the parts, seems like it's about $500 retail. You don't need to change the cowling, which is awesome (the newest F25's require a cowling change to do this). Last time I checked the inventory at Yamaha's warehouses, there was only ONE left in Japan, which usually means that they're probably going to be discontinued soon. Or sometimes it just means that there isn't a big demand for the parts.
I got all my stuff from a guy out of Florida...I lucked out and got an adjustable carburetor (idle mixture adjustment screw instead of a blank hole), recoil starter assembly (complete) and a couple gaskets and screws for $250. It was all brand new, from a freight damaged motor. That's been about 4 years ago now. The recoil starters for the old style F25's are about like finding a needle in a stack of hay. They're out there.....but it's gonna take some digging to find it!
I "THINK" (key word...think meaning not positive) that the Mercury 25 4 stroke of the same era would interchange parts-wise for the recoil starter. But don't go buying parts unless you can afford to make the mistake. I didn't get any merc parts to try-it was all genuine OE Yamaha stuff. And it fits (and works) wonderfully. I left off most of the start-in-gear protection. Main reason was because I wanted to be able to yank start it if I needed to, in gear. It's worked great and bad all at the same time. A river that I love to fish, commonly the current will be dead slow one minute and then 30 min later the water will literally rise 8 feet (in 30 min time) and the current is about 12 mph. I've been in situations where I didn't notice the water rising and the current has pushed me back almost into a dock on the side of the river. So I just grab the rope and pull, and it takes off without having to wait for getting it into neutral, push the button, put it into gear, and then take off. Saves a little time. But then once my girlfriend was with me and actually hit another dock (with the boat, bow facing upriver). I grabbed the rope and pulled but I didn't think to throttle it back to idle. It was about 1/2 throttle. It started....she went overboard before I could get it controlled. Luckily at that point of the river it was maybe a foot to 18" deep, so she just stood up (though it was ice cold). So I highly suggest getting ALL of the parts. I'm going to put mine all back on eventually.
And if anyone wonders how hard it is to start? I don't even turn around to start it from sitting on the rear seat. Grab the rope handle and pull. VERY easy to crank it over! Yamaha has a built-in compression release which works most wonderfully! Speaking of which....a valve adjustment on these motors does make them run better and start easier. I did mine not long after I got it and it made a noticeable difference in idle quality and top end power. And it's easy to do.
Good luck! I may grab a photo or two and post 'em here. The hardest part was just finding the recoil starter assembly. Once I found it all, bolting it on was pretty straightforward.