Its not illegal to change an engine DIY and get it signed off afterwards by a suitably qualified person.
The home builts do it all the time.
Its on a permit there are no issues if they did change it themselves and then got a sign off afterwards.
And to be honest if they had done it themselves it would be a big tick in my book because its obviously had someone that is that way inclined giving it TLC. More so than if it was getting put into a shop only when required.