The engine in question is an absolutely terrible choice. If you're going to switch to a 4-stroke, at least go with a higher performance 4-stroke. The 350 isn't a very well designed engine. Of the 4 that I've dealt with, I've twisted a crank, a friend of mine twisted a crank, and another guy completely destroyed his engine. Only mine was due to over-revving. Get something more capable and reliable than that.
If you must do an engine swap, go with a 125cc or larger 2t dirt bike engine or a 250cc or larger 4t dirt bike engine. To save yourself a headache, allocate the money for the swap into getting the Blaster engine built properly and it'll last for years.
I've been on the same Banshee engine since 2009, and I have absolutely tormented that engine. I've put that engine through things that I'd scream at other people for. It has only lost 2psi on one cylinder during that time. If you take care of a well designed 2-stroke (which a Blaster is), it will last.