Hi. I have an old John Deere riding mower, model SX85. The Briggs & Stratton engine is model 28M707-1162-E1. The last time I used it, it was hard to get started and then today it wouldn't stay started. It would turn over, FINALLY catch and start, run noisily for a few moments and then stall out with a puff of smoke. I checked the air filter and it was soaked with oil and other oil was in there. The oil was not refilled lately and there was the proper amount of oil in the oil tank. I went out and bought a new air filter and the same thing happened. Would this be a fixable problem, or do I need to find a replacement engine. They don't sell this model engine anymore.