Was the Cheetah based on the F-21 Kfir?.

No it's based on the Mirage 3, and it has a strong resemblance to the Israeli Kfir

The systems in any airframe will be as good as the govt is willing to pay for, however the capabilities of any 1950's airframe design are going to be severely limited by comparison with a 1970's purpose designed agile airframe like the F-16, back in the 50's when the Mirage was designed the only objective was to go very fast in a straight line. See also BAC Lightning, Lockheed F-104 and MiG 21 for similar examples.

The Cheetah was based on the Mirage-3 airframe, but it's design has beter wings, engines and canard foreplanes, It will outmaneuver a F-16 anyday

The Cheetah has an enhanced airframe alright. The drooped leading edges, sawtooth and small canards all serve to enhance its agility over the original Mirage design, I think the Cheetah is also more agile than the Kfir, taking the evolution of the airframe a stage further than IAI did, however this is still a long way short of being able to outfly an F-16.

If these mods truly resulted in a super agile fighter that can beat the F-16 then France would have adopted the Mirage IIING, which instead remained a prototype, rather than spending all that cash on the Mirage 2000 (not to mention Rafale).

[edit on 3-12-2006 by waynos]

Cheetah C (most advanced Cheetah varient) had some advanced avionics right enough, some of which were reputed to be very advanced and still classified on their retirement in 2006, particularly its helmet mounted sight/display, radar and jamming/self defence avionics, missile systems.The South African military industrial sector did an amazing job in keeping a 2nd generation fighter design competitive into the 21st century. Its various airframe developements vastly improved on the mirage 3 it was based on. It was a formidable light strike fighter. If the political system in South Africa hadn't denied them access to a more powerful engine it would have been better still. Then again if South africa had been able they'd have probably bought Mirage 2000 in the 80's/90's to replace Mirage 3, rather than completely re-work the design to keep it competitve (the airframes were practically re-built back to zero hours and airframes were illegally sourced additionally,in Israel)
However, while the Cheetah was a remarkable achievement, and more agile by far than the base Mirage 3. To suggest it could out fly a modern, unstable FBW design like the F-16 (or indeed the Mirage 2000) is a little unrealistic. That said, a Cheetah C with its advanced systems flown by a well trained pilot would have more than a fighting chance in an A2A engagement with either of these aircraft.