You should be able to jack up the trailer with the boat on it with an automobile jack. Then you can replace the flat trailer tire (trailer tires are not the same as car tires BTW).
Once you get a towing hitch you could then take your M26 to the water and work on the center board with the boat in shallow water. This should be a lot easier then jacking the boat up on land.
To jack the boat up on land you would need at least 4 jack stands to aid in the process. You'd also need a pile of small pieces of lumber to build up support directly on the bottom of the boat as it gets raised by the jack stands/jack.
In either case you need to make sure that the water ballast chamber is empty.
If your M26 is from 1993 then it is most likely an "S" model.
MACGREGOR 26S sailboat specifications and details on sailboatdata.com