It is fairly easy to mogrify the PKGBUILD only a few commands are needed.
Our inhouse packager notified me that one should be able to skip the step modifying the PKGBUILD - using -A argument - thus this should do it
sudo pacman -Syu base-devel git micro
git clone https://aur.archlinux.org/sylpheed.git
cd sylpheed
makepkg -Asrcif
If you rather edit the PKGBUILD
The line reading
arch=(x86_64)
Change to and save the file
arch=(aarch64)
Then run
makepkg -is