Many things could cause a magnet to split. Did it crack in half, or was there a glue line from MFG? I would think that it would be a break based on the context you asked in. There are 3 things that I can think of; 1. over revving, especially with the wheels free-wheeling (no load). 2. mechanical shock. 3. fatigue. 4. mfg. defect, although since you have been running this for a while, that is less likely the case.
You could e-mail or call the manufacturer or distributor for the product in your country and ask if there is a replacement part number you can order.
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.