Welcome to the fun and frustration of the hobby devious.
If a schmuck built and sold you a car, you already know that frustration...
Thread-lock - Almost all metal to metal screws and grubs require thread-lock.
One exception - the screws holding in the cooling head.
Yes, I realize that your head came off, but that could be due to improper tightening process by the previous owner.
Don't listen to competing advertised claims of horsepower, there's a lot of subjectivity and variables when it comes to this aspect of the sport.
All three engines that you mentioned are good engines and have their fans.
Personally, I prefer Novarossi engines and also run several Picco's.
A lot of guys on this site like their LRP's.
If the bearings feel smooth and inside of the cylinder has good even wear, no scratches, and good compression, I'd try and save your current engine before buying another one.
Worst case, you'll learn a thing or two while you decide which engine you want to buy.