1. Visitors to this historic site are fewer than came last year.
Visitors to this historic site are fewer than last year. works better as "visitors" to somewhere "come".
2. It is reported that vicious crimes in this city are fewer than occurred last year.
It is reported that vicious crimes in this city are fewer than last year. - better
3. The car repair workshop is bigger than appears from the outside. - ok
4. They are richer than appears at first sight.
They are richer than they appear at first sight.
5. He is running faster now than seems possible. -ok
6. He worries more than seems necessary. - ok