Hello Wilhelm. I can't suggest a way of doing it without cutting down the cover, other than making a new cover (out of plywood, HDPE sheet, or GRP, perhaps?) for the remaining opening, and saving the original cover off the boat in case you want to revert to it at a later date.
The previous owner of my boat cut down the cover, which works in a practical but not aesthetic sense. He did not, however seal the exposed cut open end of the sandwich construction of the cover, which I've been intending to correct but not yet done.
My winch is the horizontal axis type, mounted to the port side on top of a wooden platform perhaps 2.5cm thick, and drops and picks up the 7mm chain into/out of the chain locker without a problem (unlike some previous boats I've had, which required the pile of chain to be toppled manually several times while it was being retrieved to avoid jamming up).
If I was starting from scratch I'd put the winch in a similar position (though I'd also consider a vertical axis type winch), use 6mm chain to save weight, make up a new cover for the remaining opening, and store the old cover 'just in case'.
The shortcoming of the installation I have is that it doesn't leave enough distance between the winch and bow roller for the length of shank of an adequate sized anchor to allow the anchor to sit properly housed in the standard (?) bow roller when not deployed. As result my anchor is supported only part way along, with much of the anchor cantilevered forward, which is not very satisfactory in various ways. I'm pondering installing a stub bowsprit (or short bow platform or extended bow roller) to better mount the anchor when it isn't in use, and to keep it away from the hull when it is being launched and retrieved. This may need the winch to be mounted higher, depending on how that 'bowsprit' is attached to the boat.
p.s.Having said all that, it now occurs to me that perhaps another way of doing it (and keeping the whole of the original cover in place) is to mount the winch so that the chain exit from it is just slightly behind the back of the anchor locker, and fit a tube or pipe* to take the chain, at a shallow angle from the vertical, through the deck and the aft bulkhead of the chain locker. Such a tube/pipe needs to be amply wide enough to take the intended chain size to avoid jamming when it doesn't run exactly straight, and have some sort of plug at its entry to reduce the ingress of water in rough conditions. I would want to experiment with a mock-up of the pipe and anchor locker before fitting it to the boat to ensure the chain will flow, stow and retrieve smoothly without manual intervention (it's such a nuisance if it doesn't do that). I suspect this would be more likely to be successful the smaller size the chain is. (*often misnamed a 'hawse pipe', but I can't remember the correct term)
p.p.s I have assumed that you have the larger, roughly triangular shaped cover, rather than the small circular cover that some early LM27s had. The small circular cover might leave enough room for a winch and chain entry to one side, while leaving the cover intact and in place.
We have a windlass on our lm27 and have a pair of polycarbonate doors to cover the anchor locker. When we fist had the boat it was a plywood lift off hatch. There is a cut away at the aft end to allow the chain through but we tend to leave the doors open when using the anchor. he photo doesn't show the aft end but the idea works well.