MRC is a good brand. AC transformers are available from MTH, K-Line and Lionel. Older O and S trains require more wattage, so when buying an AC transformer, try to get one that puts out at least 75 watts. Check your current transformer for rust, corrosion, damage and frayed wires. If the household current wire is shot, there is rust on it or it is otherwise damaged, do NOT use it.
Replacement is affordable.
Many American Flyer trannies are also worth the work. Most Marx trannies had no built-in circuit breaker. Only a few over 100 watts had them. For safety's sake, get a transformer with a circuit breaker AND please read the instructions carefully. Store it carefully, in a place that is neither too dry nor too humid.