(Adds quotes, details) By Martyn Herman RIO DE JANEIRO, Aug 19 (Reuters) - An ambition he scrawled on a wall of the family garage as bike-mad teenager was realised by American Connor Fields on Friday as he won the gold medal in the Olympic BMX competition. If only my 14-year-old self could see this now." Fields, 24 next month, suffered disappointment four years ago in London when, after dominating his semi-final runs, he failed to finish on the podium. This time, with Australians Anthony Dean and Sam Willoughby setting a hot pace in the semis and looking favourites for gold, Fields stole their thunder with a perfect ride in the one-off climax to a thrilling couple of days. More...