I'd not even consider an AMP system, it'll be a ton of work for very little gain.
Even ported stuff than could use multi-core wouldn't run so it'd have to be heavily modified.

SMP is the only sensible way to go. The "Q-Box" type solution proposed (many) years ago is still a solution. It involves what is essentially a different OS with existing MorphOS running in a box or VM of sorts.
The problem is a new OS is a massive undertaking and that's not to mention whether you want to run on a different architecture.

Everything is multicore these days. I can get a MK809 III off eBay for 37 Euros (including postage). That's a quad core 1.6GHz with 3GB RAM and 8GB flash. Even watches are dual core (no I'm not joking!).