A couple things I'd like to add here is, whatever engine kit you decide on you will be well served to chunk the supplied mounting hardware studs and replace them with some quality hardware like the kit that Sick bike parts sells, its a well spent $9.95 for sure.
SBP, has a few other things that can make installing the engine easier also, I dont want to give all the credit to just one vendor here because we have several that are very good and I have had great experiences with, spend a good bit of time on the forum here reading and use the search feature to find specifics on what you have questions about, yeah there is a lot of hooplah.....
in some of the threads but most have some valuable nuggets strung through them also, and several of us guys on here are normally quick to try our best to point someone in the right direction with info that is usually based on our own experiences with technique and different parts and ways to make the parts work as good as they can.
Ok enough of that, now for a couple more of my Texas country fella long winded opinions and thoughts....LOL!
Also I recommend using some Blue thread locker/loctite on all the fasteners and I personally use nyloc nuts on the mounting studs and also double nut them, I have had to many issues with loosing nuts that vibrated off or lose and using crappy kit hardware can get you in a bind as well, I have had a couple rear mounting studs break when trying to get by with the soft kit supplied stuff, the loctite allows you to get the nuts and studs tight and secure without having to over tighten them and stress them to the point that vibration takes a toll on them and they eventually snap.
These things are just good insurance in my opinion and add to overall reliability.
Also make sure you have a small propane torch and a small roll of fine wire solder, when you go to tune the carbs that always come with main jets that are to big you will want to save some money and trouble in my opinion by just soldering and re-drilling out the main jet to get the engines purring like a kitten instead of blabbering down the road spitting way to much pre mix and running like your dragging an 80lb bag of cement mix...
You'll need some wire gauge drill bits for this task and it is a very simple thing to do.
Here is where I get my bits and the small bit vise to do the job, cheap tools and a simple job to do.
As Dave mentioned it always a good deal to clean the burs up in the intake and exhaust ports, below is another leak to an inexpensive set of drum sanders that will help you get that done with use in a dremel type rotary tool.
Use good thorn resistant tubes in the tire, grease the bearings in the hubs of the wheels with a good quality grease such as Lucas High Tack Red.
If you want to make a single speed cruiser bike much easier to peddle I like to replace the sprocket of the cranks with a smaller 36T rather than the big 44T and on the coster brake wheels I will even replace the sprocket on the wheel and move it up a few teeth to a 23 or 24T, this makes the bike way easier to peddle and makes it more like a multi speed bike that is in a very comfortable gear.
Hope some of this ifo and the parts links will help you out a bit.