Since you'll have 10 pieces, I suggest you just replace both transistors. Yes, we could go into troubleshooting mode using a meter to diagnose if it's just 1 of the 2 that failed or whatever, but why bother.
I just checked on eBay and it appears if you bide your time you can buy a 50-1013 working unit for around $30 (or less). But bidding/buying on eBay is not for everyone. I get that.
So if going the DIY route, when you get the parts, if it seems too daunting a task, I will replace them for you. All I'd ask is you pay for shipping. My understanding is this remote-control lockon is no longer available (at least the MTH site availability page suggests that). So if this is what's stopping your from enjoying your trains at age 92 (wow!) I might be able to help...