Some days the vocal cords just don't work.

but if you could hit those note 20 years ago - you can hit them now
I wish that were true for me. Unfortunately, the doctor that ran off with my thyroid gland saw to it that hitting notes of yesteryear will forever remain but a dream.
Some 20 years ago, I recorded a version of Paul Simon's America. It was my first attempt to use computer multitracking with a program I found called "ProTools Free" (2001). The recording was made on a Windows98 machine with a 486 processor. There was no audio interface, just a basic Soundblaster card. I managed to get some ok harmony and acoustic guitar recorded with my EV electret condenser mic and old Yamaha acoustic. Not great, but it was more "proof of concept" anyway. It would be a few years before I sprung for the Yamaha AW16G.

So yesterday, after listening to a copy of that recording, I sat down with all my new gear and thought it might be nice to redo it. There was no problem getting a respectable copy of the guitar track. But when it came to the vocals, it hit me just how much 20 years takes off of your vocal range. Lordy it's tough to try to do Art Garfunkel's harmonies. After 2 1/2 hours of trying, I had to take a break.

Maybe I just need to go back to the original and listen to what I was singing on that version. I don't remember it being all that tough. I tried a few things, drinking juice, mint, a little steam to open the head. It just didn't work! Maybe if I record the main vocal and guitar normally and then drag the speed control down on Reaper by 10 or 15% I can get into range for the harmony.

Has anyone else tried this? Does it sound unnatural?

Anyone have a time machine I can borrow?
Play to your strengths. I could never sing a raw rock blues vocal, but I have a fine tuneful voice for soft gentle vocal.
I haven't gotten back to that project yet. I did, however, find some lyrics that I had written many years ago, while I was traveling for work. Whenever I traveled by car, I took a guitar. I remember watching a little kid and her grandfather and somehow it struck me. I went back to the hotel and ended up writing some lyrics and working on chords. It was filed away on a flash drive. The original melody is long lost, but that shouldn't be a problem. Since I had the mics set up and the acoustic was right there, I fired it up and did a couple of runs, even added a few lyrics.

I'm not quite the prolific lyricist that Manslick seems to be. but it's a start. We'll see if things come together at some point.
Vocal folds need to be flexible to sing high. Do exercises in the falsetto and falsetto 2 ranges. Search for Justin Stoney online. He has any number of free vids to help improve your highs. Like stretching any muscle, stretching the vocal folds takes time and being consistent with the technique exercises. When I started B3 or C4 was about the top. 5 years later F4 is comfortable (still in my mix voice). I'm well past retirement age.