You may have limited success with a hypodermic as the epoxy in between the scarf-jointed plank ends will inhibit a proper bond with fresh stuff. Besides, being cracked, the joints are contaminated already & need to be cleaned well before any further insult's imposed on 'em.
Better proposition I'd think would have you routering out the scarf seams to at least half the depth of the planking then "injecting" fresh epoxy to saturate the newly-exposed end grain of the planking, soon to be followed with suitably thickened epoxy to fill the routered seam gap.
Don't go all the way through though 'cause then there's nothing holding the epoxy back from simply running down in the inside of your planking.
This kind of thing is common for decks and hulls but instead of thickened epoxy a wood spline is epoxied in place into a saw kerf that 'freshens' the joint surfaces. In your case the scarfs aren't long enough to deploy a circular saw for the kerfing, why I suggest a 1/8" router bit.
Find a good source for carbide 1/8" diameter router bits or better yet
up-twist carbide end-mills. Notorious for being easily broken but superb for the operation being contemplated. End mills are tougher than single-edge router bits and don't clog as quickly from the stuff being removed. The up-twist are essentially self-cleaning but be prepared with proper PPE 'cause you'll be getting all the dust they kick back into your face.
And don't force 'em. They'll do the job but if you force the feed they'll break well before they dull.
Or haul your boat out & strip off the epoxy paint.
But that won't do much for the epoxified scarfs, will it?
Can you post a pic or two of one of your scarfed planks that's popped? That'd be of help knowing what it is your facing.