Electrics

Agree by and large with what's already been said, but just suggest a quick sanity check before you spend a load of money... Obviously all the bits you have seem to be working as you say you have a weak spark. So the most obvious thing to ask first is, are they all properly connected together? By which I mean, have you disconnected every part of the ignition system and checked the connections are clean, bright, firmly located and so on, and are the contact points clean and smooth or black, pitted and corroded? It's unlikely every single past of the electrical circuit has gone bad all at the same time, so most likely you've got one particular connection that is high resistance and hindering the current getting through. Start at the coil and check the lead is good to the distributor. Usually the cable is just a push fit into the end of the coil so just pull it out and check the metal is clean and bright. Maybe spread the tabs of the connector a little to ensure a good contact. Then the same with all the HT leads, and then the contact points, and then the plugs. Have you got the right plugs with the correct gap? And are they clean and greyish, or black and sooty? You're going to have to disconnect this lot anyway if you replace everything, so you may as well check the connections as you go, might save yourself some cash.