Can I repurpose most of the components of this Prusa clone for a Delta? I have a generic Prusa clone that I'm not using (my primary issue with it is the lack of rigidity in the frame housing as built). Will the motors (ks42sth40-1204a) and controller that come with it be good for a large-ish Kossel? Something no smaller than 200x200, preferably closer to 300x300? How do I reason about this?

Also - I know the Duet board seems to be recommended for Deltas, but can I use the RAMPS board that I already have here?

I don't have a design finalized, but have been looking at this Kossel, Building a Large Kossel Delta Printer – pt. 1: Parts and Planning, which uses a Flex3Drive effector.
Stepper motors
For equivalence, 4.8 kg⋅cm is 0.471 N⋅m or 47 N⋅cm.
Looking at the RepRapWiki - Nema17, the most common steppers are:

Kysan 1124090/42BYGH4803 (54.0 N⋅cm),
Rattm 17HS8401 (52 N⋅cm), and
Wantai 42BYGHW609 (39.2 N⋅cm).

The Kossel that you refer to is of a similar size to the Kossel XL. Again looking at the RepRapWiki - Kossel, the recommended stepper is the Kysan 1124090 Nema 17 Stepper Motor which has a Holding Torque of 5.5 kg⋅cm.
So, without knowing the exact make and model of your stepper motors and assuming that the specifications for your stepper motors given in the Prusa clone are correct, your steppers are not as strong as those recommended (apart from the Wantai). However, the recommended steppers may be over-engineered and provide [much?] more torque than is strictly necessary. After all, the holding torque of your steppers isn't that much below the recommended values.
If I were you I'd build the Kossel using your steppers and it might, in all likely hood, work out fine. FWIW, I have used the Rattm 17HS8401 in my Kossel Mini and Kossel XL. I got them on eBay and were quite reasonably priced.
You should probably read this article, RepRapWiki - Choosing stepper for a delta. Whilst no concrete values for holding torque are given, this is interesting:

A good practical setup
The Fisher, a small delta printer was designed by late RRP company. As for all their printers, they were using small and compact steppers with a torque of 2.2 kg.cm. This is lower than most repraps but is sufficient if there is no mechanical problem (friction).
These small motors have a low nominal static torque, but they also have a low inductance (2.5 mH), while due to their small size, the nominal current remains reasonable (1.2A).

as is

High inductance motors
You find on the market steppers sold for 3D printers, with a torque ranging from 2.6 to 4.4 kg.cm and a current of 0.4 A. This low current appeal builders as it make the electronic driver heating much less.
However, it came at a cost, which is a very high inductance which varies from 30 to 35 mH. That means these motors are totally incapable of any speed. They are unusable for a delta and a bad choice for another printer. As an example, a 4.4 kg.cm motor wired for this low current, while having a static torque twice the Fisher motors, simply cannot reach the maximum speed used by the Fisher, effectively having a near zero torque over a given speed. Same motors with a winding giving a nominal current of 1.5 to 2 A will be more usable.

Controller
Also, yes, RAMPS is fine for the Kossel, although the firmware calibration is obviously different, as it is a delta and not cartesian printer. For calibration, refer to How do you calibrate a delta robot 3D printer?.
Although, as Scott Lahteine states early on in this video, How it's Made: The Marlin Firmware!, using an 8-bit controller for delta printers is pushing their capabilities somewhat.
Extruder
I'm not familiar with the Flex3Drive, but it certainly looks interesting. I have used the 3325_0, this NEMA-17 motor has an integrated Planetary gearbox with a 52/11:1 ratio. It generates 16.2 kg⋅cm of torque at 1.68 A. I wrote a short blog about it here, The extraordinary extruder.
Building Tips
Also, if you are planning on building a Delta/Kossel, then I'd seriously recommend watching the series of videos on YouTube from BuildA3DPrinter.eu as they are extremely informative and helped me a lot, Build manual Kossel XL &amp; Kossel Mini. I just checked their website to try to see which make and model of steppers they use, in order to get an idea of the holding torque, but they seem to have stopped trading. However, their stepper motor page states the following:

The standard motor for most 3D printers including ours, the 42byghw811
from Wantai Motors.
Holding torque is 4.8 kg⋅cm or 47.1 N⋅cm. Shaft diameter is 5 mm and
stepping angle is 1.8 degrees.

So, to sum up, your steppers should be fine.