RGB-VGA Converter -- buy or build?

[ck]

I would like to build a RGB-VGA converter but I don't know how to design one. If you have a circuit diagram, I would like to obtain it.

I have an old RS6000 320H computer which has RGB 3 BNC with sync on green. It also says "separate". It also says that 1280 * 1048(something like that). The original monitor (IBM 6091-19)has H freq =63.35kHz (something like that), V = 60 Hz.

Could you help me with the circuit?
Thanks 
Chungduck Ko