Busi entered Italian Formula 3 with Trivalleto Racing in 1988, his first year in single-seaters. He moved to Benecchi Corse for 1989 and then to Piemme Motors, where he spent the most competitive years of his single-seater career.
By 1990 he was racing with Piemme and placed eighth in the Italian F3 standings. He attempted to qualify for the 1990 Monaco F3 race but failed to make the grid. In 1991 the campaign came together decisively: Busi took two victories and five podiums with Piemme Motors to claim the Italian Formula 3 championship, defeating rival Domenico Schiattarella — a driver who subsequently progressed to Formula One. Busi finished seventh at the 1991 Monaco Formula 3 race, an international appearance that confirmed his status in that year's field. He also started the 1991 Macau Grand Prix with Volkswagen Motorsport but did not finish.
Encouraged by the Italian F3 title, Busi stepped up to International Formula 3000 with Piemme Motors for 1992, driving a Ralt RT24 fitted with a Lytton and Pearce Cosworth engine. The campaign did not yield results: he contested seven of the ten rounds but failed to be classified in the championship standings, and he did not return to open-wheel racing after that year.
After a year absent from competition, Busi joined Unicorse for the 1994 Japanese Touring Car Championship, driving an Alfa Romeo 155 under the Super Touring regulations the series had adopted. He entered only the final six rounds of the season and qualified to race in four of them. His best result was fourth place at Sendai Hi-Land Raceway in Round 16. He scored eight points in total and finished 23rd in the final championship standings.
He remained with Unicorse for 1995 but contested only two rounds, both held at the TI Circuit Aida, finishing tenth and thirteenth. He placed 27th overall in the 1995 standings.
In 1996 Busi raced in two European touring car series concurrently. In the Italian Superturismo Championship he drove a Peugeot 405 for Motortrend, competing across three rounds. His sole classified finish came at Mugello, where he placed ninth; he finished 25th in the championship.
He also appeared in the final round of the Spanish Supertouring Championship, racing an Opel Vectra GT. He completed one race and finished sixteenth, the last recorded competitive appearance in his career.
Busi's career traced the trajectory of an Italian single-seater talent who achieved national championship success in Formula 3 but could not convert it into a platform for advancement, finding International Formula 3000 unrewarding and filling later years with touring car appearances in Japan and continental Europe before stepping away from competition in 1996.