House Republicans, dissatisfied with the fiscal 1981 budget plan proposed by the House Budget Committee, have drafted a substitute calling for far sharper spending cuts, repeal of the new oil-import fee, and $21 billion in tax cuts.

The detailed proposal, scheduled to be unveiled Tuesday, would slash spending by $35.8 billion -- $19.3 billion more in cuts than the Budget Committee recommended -- and replace key Democratic social programs with no-strings block grants to states.

The plan also would provide for enactment of the Republican-backed Kemp-Roth tax-cut proposal -- which would slash individual income taxes by 30 percent over three years -- and for faster depreciation writeoffs for business.

The proposal, compiled by a GOP task force named by House Minority Leader John J. Rhodes (R-Ariz.), will be offered as a Republican substitute when the House takes up the fiscal 1981 budget resolution, probably after the Easter recess.

Although Republicans concede privately that the plan is unlikely to pass, party leaders are mounting a major effort to attract the support of moderate and conservative Democrats, in hopes of engineering an upset.

Sources say that if the substitute fails, a substantial number of Republicans will shift and support the Budget Committee's recommendations. The budget panel's plan was drafted with the backing of GOP members.

The new GOP substitute calls for overall government spending of $597.3 billion, compared with $612 billion in the Budget Committee's proposal, with a scant surplus of $300 million -- essentially a balanced budget.

By comparison, the Budget Committee's recommendations would produce a surplus of $2 billion. They also would retain President Carter's new oil-import fee, and earmark the expected revenues for tax cuts in 1981.

The Republican plan contains these elements:

$7.9 billion in savings by tightening eligibility requirements for long-standing Democratic entitlement programs, including modest changes in some Social Security programs and other affecting food stamps, school lunch subsidies and so forth.

$7.1 billion in reductions by eliminating Democratic categorical grants programs -- in health, education and social services -- and replacing them with broad GOP-favored block grants to states and localities.

$8.8 billion in cuts from repealing antirecession aid to cities and the public service jobs program and requiring states to choose between forgoing general revenue-sharing grants or an equivalent amount in categorical aid.

$6.5 billion in selective cuts in a broad range of programs, from elimination of the junk-mail subsidy to a slowdown in regional development aid. Unlike the budget panel, the GOP plan would not end Saturday mail delivery.

$5.4 billion in cutbacks resulting from a federal hiring freeze similar to that being advocated by the Democrats plus a mandated 17.5 percent reduction in 17 federal agencies and overhead cuts in several others.

The Republicans also would save an additional $1 billion by reducing the limits on how much money a person may earn and still not have to pay taxes on unemployment insurance benefits. That limit now is $20,000 a year.

They also would increase airway user fees.

The proposals to cut public job programs would slash the number of government-financed slots by as much as one-half, with the major reductions concentrated in public service jobs, the summer jobs programs and youth employment programs.

The GOP proposal for faster business depreciation writeoffs is the same one being backed by the Democrats -- the so-called "10-5-3" plan sponsored jointly by Reps. James R. Jones (D-Okla.) and Barber B. Conable (R-N.Y.).

The GOP move came as, separately, the White House completed preparations for formal announcement on Monday of President Carter's long-delayed proposals for $15 billion in spending cuts for fiscal 1981.

Carter had promised the cuts as part of his March 14 anti-inflation program announcement, but declined to disclose any specifics until after the March 25 New York state primary, which he lost to Sen. Edward M. Kennedy (D-Mass.).

The House Budget Committee's proposal, which is similar to Carter's package, is tied up in the House Rules Committee, where liberals are seeking to delay floor debate until after the Easter recess.

The Rules panel is expected to decide on Monday whether to allow debate this week and permit amendments by groups opposing the Budget Committee's recommendations. In any case, final action is unlikely to come until after the recess.