[Bunky]

I'm wanting to move up a little in power from my 9.8 and I came across a couple of early 70's Mercury 20 1 has no spark, Are the electronic ignitions in these motor expensive to work on ????

[Laddies]

Yes 70 and newer are CD of some type and Yes it can be costly

[8075mercury]

In my opinion the red stripe 1970 and 1971 20hp ignitions are the most expensive. The "aluminum donut" stator with a green and orange wire out the bottom used to cost $300 a few years ago. Check ebay but these rarely show up.
The 72 blue stripe 20hp had points and the ignition module, not too bad for costs.
The 73 blue stripe 20hp up to 76 (?) had the regular switchbox and then the 77 up to 80 had the same switchbox as the 9.8/7.5.
So, given a choice I would steer clear of the red stripes. All of these had wiring (I think it was aluminum) that is prone to deteriorate.