Keen to see it too. I’m would love to make one similar to this with an old motor I have here.
https://m.youtube.com/watch?v=FAHsxNwDLek
You can do that but i advise you not to build it because it will be more expensive if you buy this one i bought.
I also considered to do what you think and i have big electric motors at home and i also have 3 phase current at home ,380V not just 220 v .
The motor on Mac Allister has 2800 watts( 3 hp) and its really good with branches quite thick.
It pulls the branches by itself,you dont have to push them and you make economy of electric power because while it draws the branches by himself ,you can select otther branches to feed to the machine.
It has over current protection so it will stop after 10 seconds its been jammed but i couldnt jam it with any branches of hard wood,dry or wet dhat enters that hole.
150 dollars well worth and verry cheap for what it can do.
Pays for itself in just a day.
If you buy bearings,the electric power used to weld it and the electrodes,the time,the belts ,they payment for the lathe work ( in case you dont have a lathe at home) => you will make a machine that will cost more than the Mac Allister.
In Australia you have Ozito brand wich was bought by Einhell and they make these wood shredders too but slightly more expensive than Mac Allister thogh less than half the price of Bosch.I would buy Einhell/ Ozito in case i wouldnt find Mac Allister.