Hi RobSA
The work and $$$ you put into roller rockers would be well spent elsewhere (IMO). There seem to be various opinions about their worth but I think you could make a hot street car without them. The time spent on "blueprinting" the valve train would be a much better investment, coupled with some mild port work like matching manifolds carefully and smoothing out casting bumps, ridges with a small grinder all pay dividends. Make sure that there are no sharp edges in the "bowl" area behind the valves, esp. the inlet valve. All this would pay big dividends and wouldn't come near the cost of roller rockers. also match your piston/conrod weights very carefully, try to get them to within 1 gram of each other.
If you have the cast iron head it might be worth investigating if the larger valves from the aluminium head motor would fit. Doing this coupled with a set of extractors and a nice carby set up with balanced crank/flywheel as mentioned by bellett 65will make a sweet motor. A 34 DMS weber conversion also paid big dividends but I think it makes YP a bit thirstier. If you like I can take some close up photos of the set up for you. The pleasant thing about this is that cables, linkages require very little modification.
Brett's idea of the oversize Gemini pistons is great, the conversion to standard 82 mm gave more torque, so going out further would suit the maxim "More cubic inches, more power, more power, more fun" 
I have a supplier in Hobart who sourced a new timing chain for me a few years back, am getting them to find me another this week. If you want one give me a yell and I can arrange. however other members will be able to throw more light on this as chains seem to be very interchangeable, given profile, joining links etc.
Good luck, keep us updated
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I don't mind paying for the job to be done right but no one I know will touch it. They [i]"don't like"[/i] working on old cars.
