The MVVS .15 looks to be a really strong running engine. I have a couple of them, but I haven't run them yet to be sure.
Although the Norvel looks good too. But the Fox .15 makes for a great sport engine though. All of these engines are easy to get parts for too.
If your budget is tight, you can trade in a clunker engine to Fox and get the Fox .15 engine at 1/2 off the list price too. That does tend to make it most attractive then.