Valiant 318 problems

SOunds like a fuel problem. First, the sudden change could be a clogged/dirty carb. Second, a fine tune to the carb is probably in order. An electric choke is an excellent addition to the carb. You can raid parts from a Edelbrock or purchase new. You'll just need the rod and E-choke with screws to mount it on the side. Set it forget it. No real advantage in a manual choke.