Yup. What you would need is a donor G key from another instrument which could be modified to work with that horn. THAT is an INTERESTING Schenklaars.....as your (Stephen's) review notes in the postcript you added at the end....almost everything about this model is JK, and looking at all of the details, IMHO this was NOT manufactured by Schenklaars at all...because your typical JK-ish Schenklaars-assembled horns usually have some very clear details which ARE JK but also several which are absolutely NOT JK (such as pinky table, bellbrace, and keyguards)...while this one - as Steve's friend noted - is really a dead-on Couf II.
I would therefore lean towards this being mfr'd by JK, 100% and just branded Schenklaars. Or certainly all parts mfr'd by JK and at most ASSEMBLED by Schenklaars.
Perhaps the latter company's attempt to offer an elevated, 'pro' model during this period (?) had them contract JK for complete fabrication.
Problem here is - Rayan is in Saudi Arabia...I am not sure how many woodwind/reed/band instrument repair technicians there are near you, Rayan. To replace the missing key would really require someone with some experience in repair/servicing - it is NOT a do-it-yourself repair.
However, ONCE the G key situation WAS resolved, getting the sax playing again COULD conceivably be a do-it-yourself endeavor (albeit a time-consuming one, which would require a fair amount of online research and some trial-and-error).
If you were here, I'd say 'send her over to me' I could resuscitate her for around $400-500usd, into full playing shape. But....
As the horn now is, it would be worth around $150-175 on eBay (US). In completely serviced & playable condition, it would not be worth what a Couf II would be (due to the Schenklaars branding)...but it would probably fetch around $700usd.