Clamshell heads are not dangerous because of the babbitt bearings. They are dangerous because their design relies on the clamping strength of bolts that stretch over time and use, allowing a just enough wiggle room to throw a blade. It's catastrophic when it happens. I've purchased a jointer that had this catastrophic failure, and I've read about quite a few others from my owwm brethren. You are absolutely right in wanting to change the clamshell head. They are simply dangerous, and if you've seen the carnage up close and personal, you wouldn't want to be in the building when one of these lets loose.
Babbitt bearings are excellent bearings. I have several babbitt machines, and in my opinion, they are every bit as good, if not better, than top end ball bearings. If you maintain them properly with oil, they will last a century before wearing out. Repouring babbitt for a new cutterhead is not a big job, and a lot easier than it sounds. Plenty of good youtube video's on Keith Rucker's channel about pouring babbitt.
Machining a head for a babbitt machine will cost more because there is more overall length involved, as a babbitt machine would incorporate an extra 6" or so, at each end, not found on a ball bearing machine. Like Dave said, the best way to not be surprised is to send photo's and proper measurements to the maker of your choice for price quotes.
If it were mine, I'd machine a 4 knife straight head for that machine, restore it, and have a lifetime machine for the shop. I know, because I already have done this to 6 jointers. Great machines when they are set up right. The species of timber I work doesn't warrant the extra expense of a spiral head. YMMV. I paid $400 for my current 16" jointer. It was a train wreck, ready for the scrap yard. I resurrected it and now it's a perfect machine. Hard to beat 2000 lbs. of cast iron for stability in a jointer.