There are things over and above assembly that make/break this job. Nothing hard, just easy to do silly things that could cause your rings not to bed in for example.
Just better to post here before you do the job rather than "here's my pistons and cylinders fitted, isn't it handy that they come ready assembled? I dribbled a load of 20w50 oil between the pistons and cylinders". "I started it up and let it idle for 1/2 hour." That lot will get you a Doh!
How to rebuild your Volkswagen Air Cooled Engine by Tom Wilson is a good read for what you have to do.
The ready assembled bit is just so foreign made P&Cs count for taxation reasons as "Assembled in 'Murica."
