This is not an easy decision
A diesel conversion will run you upwards of $50,000. Remember, you must replace the genset (if you have one), too, because you cannot mix gas and diesel powered engines in the same engine room - that's against the ABYC and USCG rules, unless EVERYTHING in the engine room is ignition protected (diesels are usually NOT!)
You need to burn a LOT of gas to make that back.
It WILL increase the value of the boat, but not by the amount you invest - not even close.
You can buy TEN new, in-crate gas engines for what it will cost you to repower.
Repowering with diesels will vastly extend your range, improve performance, lower your fuel burn and the fuel is much less expensive. On the contrary side maintenance is far more involved and expensive with diesels (if you farm it out), and the consequences of neglecting it are WAY more expensive.
If you want to repower, don't do it on economics. Do it because you want to own the boat for 10 or more years AND you want the other advantages of diesel power.
Agree in part, disagree in others....
I don't know about the "go maximum electronic" stuff....
Electronics and salt water don't mix all that well. And being offshore without engines as a result of a "brain box" dying is not my idea of a good time.
With that said, you are stuck with this to some extent with most current engines, whether gas or diesel, and of course gas engines rely on power to run in any event, even if they're old-style "points and condensor" motors.
I really LIKE my old Jimmys. They're loud, burn more fuel, and are nowhere near as "refined" as newer diesels. BUT, having suffered an electrical failure 60 nautical miles offshore, and having gotten home without fanfare or incident when it happened......
This much I know for certain - at best in that scenario with "newfangled electronic engines" I would have been coming in on one, and I would definitely not have enjoyed the "extra time" that required. Worst case, BOTH would have shut down and I would have been screwed.
Ever contemplated calling for a tow 60nm offshore? Assuming you can rig something to get radio power (we could have managed that without too much trouble), that would have resulted in a very interesting bill - towing "insurance" or no (and yes, I have it), they won't necessarily come get you that far out.
I have no problem with electronic ignition, as it rarely fails, and if one does, and the other won't start, you can play "swap components" to get one working motor.
However, fully-electronic engines are another matter, and IMHO require careful consideration of where you're boating, what spares you can carry (and afford) with you, where the critical failures are, and how YOU can diagnose enough out there to get home if something goes wrong.
More metal, longer life....
.... is indeed a good rule of thumb.
If you're happy with the performance of the boat as it stands, I'd replace what you have. Much less pain than screwing around with a conversion.
The pain comes if you're NOT happy with what you have now in terms of performance....
Read this Months Issue of Soundings
There was a long article on a 31' Bertram which converted from gas to diesel.
For your situation though, I agree with others that the best bet would be to rebuild your current motors.
Still would probably rebuild....
... especially if you have to buy TWO new ones!
Were it me, I'd pull 'em and take 'em home, and rebuild them myself in my GARAGE! Then cart them over to a dyno shop, have them checked out under full load for a while, and then reinstall.
But that's me.... I'm funny like that, and my time is cheap and paying someone unknown is dear ;)