Hi,
I too needed to sort out the extruder, etc on my XYZ Jr. I am happy to carry on with standard reels and firmware at present .
(I am building a Delta printer to give me that freedom later on but still need the Jr to complete that task.)
All I have done is to remove the existing extruder assembly and attach a new front end - a Mk 8 extruder frame ( L.H. sided) on to the existing stepper motor - no wiring mods other than removing the white 4 core lead from the sensor on the old unit. I mounted it all in the same place as the original using a flat metal strip and a printed fixture similar to Thingiverse /thing/1720214.
To make sure everything kept cool I fitted a 12v 50mm fan above the extruder to blow cooling air down on to it. I connected the fan in parallel with the main exhaust fan at the rear of the cabinet.
No structural mods involved and I get perfect prints every time with print times over 14 hours presenting no problems. The extruder works perfectly and the filament used indicator still works as well - my last reel let me print a small part with only 5 m indicated on the reel !
Because of the many jams I had prior to this I had resorted to drilling out the blockages in the hot end so if there was a PTFE lining it has well gone . I noticed that Maplin were selling a replacement item but when I asked they said it only fitted the Wifi Jr and not the basic model I have.
I took a chance and purchased one - it is the exact replacement but has the addition of the auto calibrate function - you can remove this with one screw or if you are using XYZ firmware use the function now on the latest menu.
Sorry it's been a bit of a long story but this has really transformed my machine into the item I had expected it to be before reality took over.
For those interested: the extruder Mk8 frame mod was £6.99 using eBay item 121805539946
Mounting bracket could use Thingiverse/thing 1720214 ( sensor ring can be removed)
Maplin Code for replacement hotend is A10YZ price £86.99
Hope all this helps and will offer an easy solution to the XYZ Junior woes.