I'm so sorry you lost him.
I also do my own post mortem exams and I actually find it helps me to deal with the loss. I'm a very emotional person (I cry at the drop of a hat

) but I'm also quite practical, so becoming focussed and absorbed by what I find inside them helps to distract me from the grief. I recommend covering the bird's head with a sock or something once you have checked inside the beak for lesions or plaques. For me the head and eyes are what I identify with so once I have that covered it is just a chicken carcass. I don't use any special tools, scalpels etc are not necessary. I have a little pair of sharp scissors and a sharp kitchen knife and I work in my poly tunnel/greenhouse rather than dissect the bird in the house. The light is good in there, it is warmer than outside and if there is any infection I'm not bringing it in the house. I work on my potting bench on an old plastic feed bag. It helps to have a bucket of warm soapy water and an old towel so that you can rinse your hands when you need to and if you are going to take photos, which is always really useful if you need help figuring out what the problem is, having your phone/camera wrapped in a plastic bag means you don't have to keep taking gloves off to pick u[p your phone for a photo. Taking shots of the organs in situ and then removed enables us to have a better idea of what we are looking at. If there are abnormalities, it is sometimes difficult to ID an organ other than by it's location in relation to other organs.
If you have processed chickens before you should have an idea of what normal looks like and which organs are where.
My gut feeling is that you will find tumours, probably lots of little ones throughout the digestive tract which may look like little white nodules or there may be one or more larger tumours which usually look a bit like white fatty tissue. The digestive tract is where I would focus my attention since his symptoms would indicate the problem is there. Be sure to check the inside of the proventriculus as well as the gizzard.
The final thing I do is to palpitate the length of the gut for large roundworms and cut open any sections where I feel something suspicious. Cutting into the intestines is the only smelly bit which is why I leave it until last. There should not be any significant blood or other bodily fluid, so don't worry about it being an overly messy process.... only if/when you cut into the gut generally although with this guy having crop problems there may be some liquid and smell in the upper digestive tract.
I hope I have reassured you that it is something that you can have a go at and along with others will be happy to help you figure out what was going on.
Good luck.