Without any Regex nor string concat, and as an enumerable (it supposes keys don't have spaces, but values can):

    public static IEnumerable<KeyValuePair<string, string>> Split(string text)
    {
        if (text == null)
            yield break;

        int keyStart = 0;
        int keyEnd = -1;
        int lastSpace = -1;
        for(int i = 0; i < text.Length; i++)
        {
            if (text[i] == ' ')
            {
                lastSpace = i;
                continue;
            }

            if (text[i] == ':')
            {
                if (lastSpace >= 0)
                {
                    yield return new KeyValuePair<string, string>(text.Substring(keyStart, keyEnd - keyStart), text.Substring(keyEnd + 1, lastSpace - keyEnd - 1));
                    keyStart = lastSpace + 1;
                }
                keyEnd = i;
                continue;
            }
        }
        if (keyEnd >= 0)
            yield return new KeyValuePair<string, string>(text.Substring(keyStart, keyEnd - keyStart), text.Substring(keyEnd + 1));
    }