There were 3 generations of the SSI modules, since yours is missing, it will be a crap shoot to get the one that was on it, but all three will work. Unless you piece meal it together yourself and know what your doing, any of the conversions out there will run you $140-160. I am going to try to put one together for around $70, hopefully
If you have to spend the money, I would recommend Dale Covert's Overnight ignition solutions kit, it's a direct plug and play and does not require the battery to run the engine. All the others you will find convert you over to an auto style battery-coil system meaning you need a good charging system also.
I have a break down of the systems in my sig.
Whatever you do, do not go to ebay and buy and overpriced 40 yr old unit that may fail on you tomorrow.
I am also working with a guy who may have a fix for burnt up coils for close to no money
Tecumsehs get a bad rap, but I love them, just have to learn their bugs. When running good, they are workers.