Be prepared and safe with an Interlock kit for your existing panel
If you have an FPE panel you could be out of luck. I looked high and low. You can change the guts to a newer brand. You will have trouble with your insurance comp. anyway. "they don't like FPE" not only are the breakers not accurate, the panel dimensions are too small also.
Challenger Generator Interlock Kit 150 or 200 amp panels
L14-30 Generator Power Cord 4 Wire 10 Gauge 125/250v 30 Amp 20 Feet
30 Amp Generator Cord, 10 Gauge Heavy Duty L14-30
PB30 30-Amp NEMA 3R Power Inlet Box for Generators Up to 7,500 Running Watts
Adapts a 4-prong 30 AMP L14-30 receptacle to two -15 receptacles
Power Inlet Box, 50-Amp for Generators Up to 12,500 Running Watts