Going from a 125 to a 750cc sportbike is doable, especially for an older sportbike that is not as ridiculously fast as the newer ones. Be extra careful with the throttle and front brake at first.
I would have a mechanic check this 1982 bike, including a compression check and a test ride. A bike this old might need a lot of parts over the next few years to keep it running, and it might be difficult to find new parts for it. You might need to search for used parts.