I’ve been thinking about buying a toploader nes that is modded to have AV ports, would that be allowed in speedrunning?

Yeah, it should be

Yeah it's fine, it's basically the same reason why RGB mods are allowed. If you decide you don't want to bother with looking for a modded US Toploader, the JP Toploader has the AV Ports without modification; the community and Ebay listings generally call that model the AV Famicom.

Thanks guys!!