Stalls when cars warm?

I recently got into an accident involving the oil pan and exhaust. Got it fixed. The problem is that it does not stay on long. The mech said that everytime the car warms up and shifts into gear the car stops running or overheats. So then he said there might be a problem with the transmission. Not sure how. But yeah the accident was supposed to be covered by icbc but the ajuster said that since the tranny has to be taken apart its up to me and if the problem isnt accident related and can be normal wear and tear icbc would not cover it. WHAT DO I DO NOW. Mech said if it comes down to the worst it'll cost about 7gs. Anyone know what the problem can be? My friend said that his friend got the same problem he replaced the transmission oil pan and refilled it with tranny oil and the car is fine. Is my mech tryin to scam me