It can be done, and not that hard. I've done 2 of them. Mine and a customer's.
Buy a complete manual starter assembly from a 98-06 Yamaha F25. Ebay is a good place to look for parted out motors, though, the rope start assemblies are more expensive than they should be, IMO. BUT if you bought it all new from Yamaha, expect to pay $600-$750 depending on whether or not you want neutral start stuff. Mine (ebay) were both under $200, about 5 years ago.
Remove timing belt cover from your electric start motor. Install the rope starter assembly. 3 bolts. I chose not to use the start-in-gear cable and linkages because of where I use my boat/motor. Customer's boat, he said he took it off-but I had to put it on (liability...). That was the hardest part, getting all the links hooked up. OH (edit
don't forget to get the ratchet piece that's bolted to the top of the flywheel. It's not that expensive from Yamaha, but if you can get the seller to let you have it for a few extra pennies, it's a wise investment. Adding recoil start to an otherwise electric start only motor is in itself a wise investment.
LUCKILY the cowlings are the same on the electric and manual start motors, up to 2007. The "newer" (08-15) are completely different. Bigger cowling, different for manual and electric start, etc. Adds more complextity and therefore cost.
Done with factory manual start assembly, it looks factory and functions as reliably as a Yamaha should. If you wanted to really do it "right", change the camshaft too as the compression release is slightly different. Also the manual start carburetor bowl is different. The carb body is the same but the bowl is different. BUT the electric start camshaft and carb work just fine with manual start. It starts beautifully as long as the carb isn't plugged with stale fuel. I usually just reach around with my left hand behind me and feel around for the rope, yank it kind of sideways and it fires right up, usually in gear in that situation. Very little effort to crank it. Half as much effort as it takes to crank the Yamaha 25 2 stroke. Maybe less than half.