Treasury Secretary James A. Baker III is considering modifications of many of the principal provisions of his department's tax-simplification plan, but has made no final, or even tentative, decisions, Treasury and congressional sources said yesterday.

Baker and Deputy Secretary Richard G. Darman, who are meeting almost daily with legislators interested in tax reform, are looking at six or seven ways to modify the department's proposed limitations on write-offs for purchases of equipment and structures.

Among them: making the current multi-year depreciation system more like a one-year write-off, preserving the current law's bunching of tax benefits in the early years of the investment's life and sticking with the department's proposal to lengthen the write-off periods but index the investment's value to inflation.

Treasury also would like to encourage new ventures by reducing tax rates on profits from investments in start-up firms, although questions of definition make that difficult. The department's simplification plan was issued last November and now is undergoing modification. It would change the tax rate for capital gains to the same rate a taxpayer would pay on ordinary income, with a top rate of 35 percent. A lower tax rate for the narrower venture-capital category is seen as a way of gaining the support of high-technology firms for simplification.

Treasury also is considering lowering the threshold at which contributions to charitable organizations become deductible. Under current law, all contributions are deductible up to 50 percent of income. Under the department's simplification plan, only contributions that exceed 2 percent of income would be deductible. The revised Treasury plan also might place an overall cap on deductions for business expenses rather than limiting them one by one, as the current proposal does.

Sources also said Baker and Darman are hoping to get the top individual tax rate below 35 percent, something Reagan has said publicly that he would like to do.

All those goals, however, cost more in federal revenue than the provisions of the original tax restructuring plan. To have the proposal bring in the same amount of revenue as current law -- as the first version did -- Baker and his Treasury colleagues may have to give up such provisions as indexing interest income and payments to account for inflation and allowing corporations to deduct half of dividends paid, observers suggested.

Treasury officials and others consider the interest provisions to be too complex to implement, and business leaders are not very enthusiastic about the dividend deductions.

Treasury officials emphasize that they are still in an "exploratory phase." Despite reports to the contrary, "no tentative decisions have been made," one official said.

Baker and Darman also are meeting almost daily with congressional sponsors of tax simplification -- plus leaders of the tax-writing committees and others -- in hopes of enlisting support for rewriting the simplification plan. Baker wants to have that rewrite ready for public release in early May, although it is not clear whether it would be released before or after President Reagan returns from his trip to Europe and the economic summit in early May.

Yesterday, Baker and Darman discussed depreciation, capital gains and charitable contributions with three of the four principal simplification sponsors: Reps. Richard Gephardt (D-Mo.) and Jack Kemp (R-N.Y.), and Sen. Bill Bradley (D-N.J.). Sen. Robert W. Kasten Jr. (R-Wis.) was unable to attend. The duo also sat down with House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) the day before, and on both days with Senate Finance Committee Chairman Bob Packwood (R-Ore.).

Proponents of simplification generally have agreed that its chances are better if Congress can be pushed to act quickly so lobbying groups have less time to gather their resources.

"There's no schedule, but there certainly is an intent, a spirit of cooperation that I would imagine reflects a desire to get something done by August," Kemp said.

Meanwhile, some of the proponents of tax revision are planning media blitzes built around the last day to file federal income-tax returns, April 15.

Bradley and Gephardt, who are sponsors of the principal Democratic simplification plan and who often are cited as presidential hopefuls, will hit the road the week before the 15th with a 12-city speaking and public-appearance tour that will wind up in New York on the 15th.