If it were me I'd be looking for a Kawi 650 twin... visualize the Verseys with MX suspension. Lots of engines around (in ninja form, if you're in the states) proven reliability with good power. I will build this bike some day, it has been on the list for quite a while.... mostly I need to find the engine local, 'cause shipping is a killer for me.
Big singles are already out there.... so what's the point doing that? Easier to upgrade suspension than graft an engine.
Edit: my "failed" attempt a couple years ago.... 440 is too small of an engine. Susupension is DR.