The frame will twist and you will be in A&E before you have a chance to use the full power of the 650, there is a reason the 650 frames are built around the engine in trellis form and the steel is stronger than what Hyosung uses on the 250 frames. So personally i won’t entertain dropping a 650 in that frame to save your body.
Why not buy a spare engine and fit it yourself, it’s not too hard , just requires a lot of time and patience but your hands are free labour.
$1000 is cheap compared to UK standards as an engine alone in the UK is actually hovering around $500-800 USD with sketchy histories by last owners lol. Then couple dollars for labour ofcourse unless doing it alone.