Does anyone know of a mod or fix for a very heavy clutch action?

Not expecting one finger action.

Has the clutch been disassembled before it got so heavy?

Bike fairly new to me and rebuilt a couple of years ago by previous owner.

Restoration seems to  have been carried out well but I have no idea what was done inside.

Everything works ok but bike has not been used since rebuild.

Try a clutch lightener fits on cable and a decent long lever make sure all parts are oiled inc cable and should be fine , cable routing also important ie no tight bends or tied to other parts restricting movement of outer cable also make sure aligned with arm correctly

I don't think you need a lightener if everything is working OK.

If after checking what zerorev3rev4 has written the clutch is still heavy, you should open the engine and check if the bronze pusher is in the right position (it can be rotated and the bike's clutch won't  work properly).

You can remove two of the clutch springs. Works great unless you plan large obstacles. Used it that way for about 5 years until our vintage club went to harder sections. The 200 was my favorite vintage  bike being underpowered as it is. But I had to go to a larger bike when the ledges got bigger.