It could be pretty spendy, you will need the starter and starter bracket hardware, solenoid, battery tray/battery, wiring, the ring gear, and maybe an ignition switch. New those parts will exceed the value of the snowmobile, IF you can find them. The easiest way to get a starter "kit" is to find a sled with a similar engine that has e-start that is wrecked or has a blown engine. That way you have all the parts necessary.
Sometimes you may find someone on ebay that has taken the starter system out of a dead sled and has it for sale, but having a sled to take it off of to reinstall it on yours is a much better training tool.
If you go to the online parts places, you won't find any e-start on the Summit 670 as it was an option. To view some of the parts, choose a sled that does have E-start and you can see the price of parts for that sled, and even then, those parts may not fit your application. One sled that has E-start standard is the 1998 Ski-Doo SLE Touring.
Good luck! It may be quicker to try to find a sled with e-start already installed...
1998 should have the wire harness in place, but you will need to add the heavier cables from the battery to the starter.
Honestly, the biggest issue is the ring gear. Finding it might be easy, but installing it might be difficult. Best bet would be to find a whole clutch with the ring gear already there, but that ups the price a lot.
The reason installing can be difficult is that the clutch has the flat spots for the ring gear, but the holes for the bolts are not there. You have to drill and tap the holes. Not to difficult until you realize that you have to center it perfectly. Even 1 mm off will make it rattle and cause wear and tear on the crank bearings and seals.
Even if you get it perfectly on there, it still might need to be balanced.