In the meantime, you can take some of the slack out of the mainsail luff by rigging, as best you can, some kind of "downhaul" or "cunningham". You could use the same fitting on the mast or deck that keeps your boom from creeping up. Use one of the luff's grommets, lowest one is better, or you may even notice a lonely grommet all by itself with no slider clipped on... that stray grommet was most likely used for that purpose. It's very common for small boats and beach cats to have non adjustable halyards... many, such as a Laser, have no halyard, the sail has a sleeved luff that slips over the mast like a curtain rod... then at the bottom of the sail a cunningham or downhaul is rigged up using loops tied into the line to give the thing extra purchase... you can do the same thing, yet it doesn't have to be as complicated because you're just setting it up to essentially shorten the luff.
Taking care of the vertical part is easy, what will be more difficult is the horizontal excess. If you're lucky there may be a reefing grommet in the leech (back edge of sail) that will allow you to thread a piece of line through it so the effort goes diagonally... down and back... to help get some of the excess out. You can be creative here.... it honestly makes no difference what it looks like... You will get more looks leaving it the way it is now, than trying to make creative changes. The other sailors will see your attempts as a guy who understands the sail doesn't fit......and you may even get some helpful, well intended suggestions we may not have thought of...
One of the other guys suggested a few more pics might help. I agree. Lay the sail out flat on the ground so we can see it entirely. Also take a picture of each corner...... NOT super close up... but a pic that shows a corner and 2-3 feet of the edges. The other shot that might help is how the panels and their seams are run... because that sail's cloth looks more like a cotton sheet... soft and pliable... so it would be pretty easy remove some ot the belly by re sewing some of those seams. A home sewing machine would be fine for the panels, and hand stiching for the new grommet on the leech is all you need... except you'll want to order some dacron thread and a couple of stainless steel grommets from a pretty cool resource... "Sailrite.com" Check out their website and ask them to send you a catalogue. They also have a lot of how to vids... you'll learn a lot. Okay.......... have fun. Sorry I got so chatty, but ....uh... that's my roll.... and I sensed your enthusiasm... which is all I need to get me going. heh, heh.