To import a public key in your keyring, you need to have that public key. Since you lost your
.gnupg folder, you also lost the local copies of the public keys you want to reimport, so you have to retrieve them "elsewhere". Public keys of other people are usually found on their Web pages, attached to signed emails they sent you, or on public key servers.
If the public keys you are after are indeed on key servers, then use
gpg --recv-keys to retrieve them and import them in your keyring. E.g.:
$ gpg --recv-keys 2EF8C903
gpg: requesting key 2EF8C903 from hkp server pool.sks-keyservers.net
gpg: key 2EF8C903: public key "Luuk Scholten <antispamsnip>" imported
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0 valid: 7 signed: 4 trust: 0-, 0q, 0n, 0m, 0f, 7u
gpg: depth: 1 valid: 4 signed: 0 trust: 3-, 0q, 0n, 0m, 1f, 0u
gpg: next trustdb check due at 2013-09-23
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
As you see, this command found exactly one matching key for the specified ID, and imported it in my keyring.
(There is no security issue in importing public keys from other people in your keyring. Importing does not mean trusting.)