Few decades ago, ACE format was very popular alternative to RAR, so you might still find many .ace archives for download, especially on websites that offer old content. You can still open/extract .ace files in modern tools and even repack them to alternative formats, including RAR format. In the case of ace to rar conversion, WinRAR is actually the most obvious choice, but any similar archives will do.
Just be careful with handling unknown ace files, considering that few years back this format was used to send malware packages over emails.
Updated: July 19, 2023