There's really not a lot of difference between a Helicoil or Timesert, they both do the same job.
Timesert are probably more DIY but if I had a choice of either I reckon I'd plump for Heliciol. Straightforward job on a plug AS LONG AS NO SWARF GETS INTO THE MOTOR. It would probably be safer if the head was removed before doing any repair.
The rot hole right through under the seat - I presume this is in the frame, or does yours have a metal seat base? If you are repairing just the seat base you may be able to patch it with a fibreglass repair kit and not disturb the sponge and cover.
The paint bubling on the frame - a time for rubbing down and curing any rust before undercoat and spraying.
It would be much better to repair the frame with a welded patch for sure. Although the frame appears quit firm and steady it is designed to flex and chemical metal repairs will only be as good as its stick - which may well be to a bit of hollw tubing with rust inside!
Stripping the bike down to the frame is quite a major job if you aren't used to spannering.
Take a look at how long Jarrah's been on renovating his bike.
The work will definitely be worth it if you are considering blasting and powder coating but it'll be out of action for some time.
