If you're on Mac or iOS (which you will be if you use Safari), the team from Kagi also develop Orion. It's a browser, heavily focused on keeping to WebKit princ

If you're on Mac or iOS (which you will be if you use Safari), the team from Kagi also develop Orion. It's a browser, heavily focused on keeping to WebKit principles, and obviously supports custom Search engines by default. It's not a 100% clone of Safari, but it's now matured far beyond a "scratching their own itch" project to enable custom search engines - it gives you the performance and power efficiency of safari, with the option to run most Firefox or Chrome extensions (by implementing the extension APIs in WebKit). The iOS version isn't quite as mature as the Mac version (which is definitely usable as your main browser), but it's getting there.