There is no way Martin beats Chretien in 1990 unless Chretien doesn't run, itself an unlikely scenario. If we handwave and say Chretien stays in retirement, then Martin would win a big majority similar to Chretien's in 1993. Martin is far less skillful politically than Chretien, as OTL conclusively proved, so there will be problems later. Being to Chretien's intraparty right and popular out West, Martin might make some Western plays later as he was expected to do in 2003 before Adscam hit and he had to play defence. Independence: Martin was disconnected from daily political operations as finance minister, though good chance he takes a deeper interest as PM. His predictions of economic calamity IOTL went over like a lead balloon even among swing voters, and a gaffe like that could be fatal to the No side if late enough. To run the No side better, Martin has to remain personally engaged without any complacency, and impose federal authority on the provincial cousins (as Pierre Trudeau did in 1980), who were almost completely useless. Martin doesn't have the personality to do that, even on someone as weak as Daniel Johnson.
Chretien would never be PM, needless to say.
Martin would probably not call a snap election during the Red River Floods as Chretien did, which was the initial trigger for Martin's hostile takeover. Perhaps not 2000 either, simply because it required a killer instinct which Mr. Dithers does not possess. (Also why Martin didn't try a caucus revolt even though by 2000 Martinites had a caucus majority) What happens with his conservative opponents depends on whether Martin makes significant Western inroads and the respective parties' strategy. Manning will not try a hostile takeover, nor will the Tories be receptive to a friendly one. If someone other than Clark succeeds Charest as PC leader, or if Charest remains PC leader, then there's a decent chance that a post-Manning Reform/Alliance leader could merge earlier. But only with Harper as CA leader. If the Tories lose official party status in 2000 then their donors will shut the lights off and force merger talks, as happened IOTL. Martin will last until conservative forces merge under a capable leader.