Roof ventilation for old house

I have a 100 year old house in central Indiana. It's below average shape, I think was flipped at least once or spottily repaired here and there. The roof was redone some time ago, although the seller had no info on when or by whom, my guess they did it themselves.

The house is a small, 1.5 story--the second floor is really a finished attic right under the unventilated roof. The actual attic is about 4 feet wide. There is a ridge vent on top, but it seems they only put it up for looks since I couldn't see any openings/holes/gaps from inside the attic. There are no soffits and no inlet vents. Also not sure how much space there is between the roof and the sloped second floor ceiling that's right under the roof to even add insulation. The roof and the second floor get ridiculously hot and I really want to remedy that.

Attached is a pic from the inside, I can take some from outside as well if would help.

What would be a good approach here? I'm waiting on estimates from contractors, but would rather DIY it if possible just because don't think any local roofers would do this for less than $1000, and I could probably do it myself for around $300 in a weekend or two. Was thinking of just removing the ridge vent and cutting the outlet holes then nailing it back on. And for the inlets to use under shingle type of inlet like SmartVent (local stores sell similar products that claim can be easily DIY'ed), or adding vented soffits and drilling vent holes, then adding insulation.

What would be a better option here? Would I need permits and get engineers/architects/inspectors involved?