I have to say, I've been undertaking the task of trying to do a retro style spy game. What I have completed of the game is not what I thought it would be. I set out to write a spy game, like James Bond rpg or like Top Secret rpg. After writing it out, and getting 100 pages in, I figured before I add anymore detail, let's have some people read it. After the readings and advice, I chopped out most of what could have lead to "too much reference to James Bond", I hacked it down to 50 pages. I Stripped out all the references to almost anything, except a general reference to the word spy or secret agent.
What I did notice is that while the game is not focused 100% on being a spy game, it is what I like to call a nice little barebones system that can be used to play ANY type of game.
So I put this theory into effect.
The system, is basically a D100 or percentile system, with an "edge factor" multiplier, in which the GM uses to increase or decrease the number. The harder the action the lower the player has to roll.
The play test group, was assigned the task of creating 3 types of characters, spy, fantasy, and sci-fi. Then play a game with each character.
After a few play sessions, and various notes, it seems it can be done. The easiest two was the Spy/Modern & Fantasy games. The challenge came with the Sci-fi game, as they wanted to do things like space wars. After some tweaking and a whole 16 pages of just game tweaks we came up with a barebones "Sci-fi" add on to the game.
Basically, I am going to run with a barebones system, with addons to it, that you can grab if you want or you can just get the barebones and do the work yourself. Your call.