https://photos.app.goo.gl/n5FAH7UrXshb8B1k9

 I'm close to taking the frame off, and I'm stuck on this electrical plug that won't come out. It's the one that runs along the temperature sensor, which was also difficult to get out, but even with two pliers this one won't come out. Any pointers?

That's the oil pressure switch lead. See the small hump and slot facing you? Take a small 'jeweller's screwdriver' and slide it under the front edge of the female connector block to lift it up far enough to pull the male connector out. If it's well and truly stuck, then cut the bugger off and replace it with a couple of 3.5mm Japanese terminals  

Actually, looking at the photo, you've inadvertently done enough damage to the wires to make cutting it off the best option.

Yea screwdriver didn't work either. Snips made quick work of it though, thanks for the response

3.5mm Japanese terminals

Complete electrical noob here, would these work?

https://www.hobbytown.com/eflite-3.5mm-gold-bullet-connector-set-w-heatshrink-3-male-3-female-efla241/p39357

Or this?

https://www.hobbytown.com/eflite-ec2-male-female-connector-set-eflaec203/p122426

Apologies - 3.5mm was a typo; it should have been 3.9mm. Search ebay for 'japanese bullet connector'. You want (preferably) this type:

https://www.ebay.com/itm/296217884721

It does mean buying the correct crimp tool but I'm willing to make a good guess that you'll find a use for more of them further down the line.

https://www.ebay.com/itm/204866624050

Damn. I wish I could have got the crimp tool that cheap in the UK!!

Got it! I appreciate the help!