I would say a really "tractory", XR-like motor at 350cc would be it, with minor brake, suspension, and swing arm upgrades (1 to 1.5 inch increase for each). Less than that and it would still bog sometimes / more and you'd need a new chassis -- meaning it wouldn't be a TW. Maybe 30 horsepower and 23 foot pounds at around 7,500/6,500 RPM... This would open up 65 mph cruising and much bigger hill climbs. Of course, IMO, this would never ever happen, but it doesn't hurt to dream.