If by 'full refresh' you mean 'complete restoration' then it's going to cost an arm and a leg!
Prohibitively expensive; in other words.
I rarely see GTOs nowadays and that only means one thing; that the market has run out of spares for the said bike.
So unless the bike possess a great sentimental value to you; I'd suggest you to get rid of this thing and get a 4-Stroke 125.
Contrary to popular beliefs; GTO125 isn't a race bike. It's not even a fast bike as it barely churns out 12HP which is comparable to CG125 and YBR125 while The GS150 makes about 1 more.
https://www.motorcyclenews.com/bike-reviews/kawasaki/kh125/1975/
KH125 = GTO 125.
So... Don't believe the hype about RX115 and GTO 125. They're just regular commuter motorcycles, no better than CG and YBR.
Good Day!