Replacing roller-clutch freehub with ratchet type
At the absolute minimum you should be able to replace just the freehub body. Some Googling should turn up some compatible options.

The plot thickens, this hub seems to use a different freehub style to most other Shimanos. This would work with the majority of clickity hubs, but Sheldon says the silent ones are different.

The freehub body is the dark grey splined part of the hub that turns.

If you remove the axel and bearings you can pop this off with a large allen key, I forget the size, 8 or 10mm.

Getting it all back together can be a pain, the freehub's easy, but the first time you set cup and cone bearings yourself will make you curse.

The next option up, is to replace the hub with one that already has the freehub you want and get someone to relace your wheel.

Then a whole new rear wheel, then a new bike.

These options go from most to least work and least to most cash.