There appears to be a flushing plug on the exhaust cover.------The tell tale comes off the exhaust cover.----Thermostat has nothing to do with the tell tale.
The 100:1 mix was a factory blunder / mistake .----Do a compression test on it.--Post the numbers.--Make sure lanyard clip is installed and working properly.----Run at 50:1 on it now.
Charge the battery and have it load tested.-----Take starter apart for inspection and ohm test on armature.----No $$$ spent to do that.---Have to ask.-----You are pushing the key in and holding it in while cranking ?
Type C was for electric shift.-----But for a few years it was used on all OMC models.----And listed on those stickers.-----Use any quality outboard gear oil in that 1968 big twin.