Being that's now a rear wheel drive setup, there isn't as much need for it to be a LSD. Personally, I would change the output shaft seals, new clutch and bearing, change the fluid, and drive it. If it was working fine and you aren't drag racing it, it should be fine. If you are throwing a ton of power at it and banging gears, yeah, then you might want to freshen it up or consider the F35 if/when they get the mounting finalized. The engine/transmission is easy to drop out of these cars and to make changes in the future should you need to.
If a builder already has their frame, the F40 is no good to them, correct? Unless they want to mod themselves. And a F40 frame may be available in the future, correct? Just trying to get the facts straight.
If you look on www.car-part.com it shows junkyards around you and you can search for transmissions with limited slip "g85 option". Around me they run around 800-900 bucks with limited slip and I've seen some with pretty low miles.
To be honest I'm not sure how you would know while driving that a goblin doesn't have a limited slip differential.
We drove Adam's supercharged goblin with an F23 for 2 years and never got in a situation that you could tell that it did not have an LSD.
If you dumped the clutch it always spun both tires.
The only way you might get it to spin one tire is to start doing a tight circle and increase the throttle until it unloads the inside tire enough to spin it but it's tough to do.
If you watch the videos of the All/Terrain it always spins both tires and it is a base model auto without an LSD.