Run some current and still-compiled-for-i686 Linux or *BSD distros on it. See what breaks, save logs & report issues upstream.

A lot of support for old CPUs & platforms they're on goes through emulators like QEMU. Or plain old 'go with specs, disturb code as little as possible, and assume (hope) it still works'. But nothing beats running on actual hardware. Uncommon configurations, specific drivers that may have been broken (sometimes for a looong time before someone notices), spec says A but hardware does B, etc.