Personally, my list of trusted engine builders is fairly short ....
Just me and Honda
Plenty of good engine builders out there , but where's the fun in letting them do it all?
I used a respected local workshop to do the rebore and valve work, the spanner work
I was happy to do things myself and take my time, reading up and using other restorers' advice etc.
Having the receipts for every bearing, seal, shells, pistons etc you have bought gives some comfort.
It's also good to retain the bike's original engine...most of it !
If some things went awry during the build they would be sorted out along the way.
Carbs and electrics make the engine look easy !
If I had spent cash on a rebuilt motor I would have wanted to see the workshop, gauge the mechanic's experience, the attention to detail, then talk to others who have bought from them
- plus see what quality of parts had been used.... The list goes on.
Cosmetics like the paint and getting every fastener etc to the original standard is important for some but tells you zero about the internals.... A hint as to the quality of the rebuild .....maybe?
A rebuilt engine needs to be heard running most would agree, so buying from the picture makes for huge risk.
Guarantee or warranty ....
.. ....doubt that.
Buying a mechanically sound complete bike or the engine from one you have seen and heard would probably be a better bet for me - only if i wanted to save time of course !