Ok, first, I have to ask what your definition of "blow up" is.
Second, if you're causing any engine damage to a blaster, in that short of a time period, there's something else involved other than you just riding it hard. We've had all kinds of 2 strokes, many that were poorly maintained, including blasters, and we would beat the crap out of them all day for years and years, and never had one come apart without something else going wrong to cause it (like running too lean or not mixing the oil right). To me it sounds like either you're doing something horribly wrong when you're rebuilding it, your jetting is way off, your oil is mixed wrong, or something like that. Blasters are one of the most reliable, indestructible 2 strokes out there.
And if you think that you're going to have better luck with that 250f simply because it's a 4 stroke, I LAUGH IN YOUR FACE!!! 4 strokes are no more reliable than 2 strokes, especially when you're pushing them to their limits, and there's a lot more to go wrong. Seems that you're either frustrated and aren't thinking about what your're saying, or you're just horribly misinformed.