House Republicans have drafted a new plan for subsidizing home buying that is more modest than one President Reagan vetoed last month, and the president has promised them he will stay neutral on it, at least for now.

The new GOP plan, drafted by Reps. Thomas Evans of Delaware and Tom Corcoran of Illinois, would take $1 billion in fiscal 1982 funding for the Synthetic Fuels Corp. and use it instead to subsidize mortgage interest rates for moderate-income home buyers.

House Democrats also want to resurrect the housing assistance issue but have not yet decided on a plan of action, a staff aide to the leadership said. The Republicans plan to try to attach their program to a fiscal 1982 supplemental appropriations bill, to be brought up in the House Appropriations Committee next week, and the Democrats might decide to use that as a vehicle for their plans, as well.

Congress had approved a mortgage interest subsidy plan overwhelmingly, with strong Democratic and Republican support in both houses, as part of another urgent supplemental appropriations bill, but the president vetoed the legislation as a "budget-busting bailout" of the housing industry. The House sustained that veto when Republican members fell in line with the president.

"This will get the Republican Party off the defensive on the issue," Evans said of the new proposal.

Evans and Corcoran took the president's pledge of neutrality, made in a private meeting with them on Monday, as a go-ahead sign on their proposal. But administration sources sought to dampen any hopes that the president ultimately would accept the idea, clearly leaving the president's veto option open.

"I took it to mean encouragement to get the measure approved by Congress," Corcoran said. "After all, he was talking to two loyal Republicans, who could have been pursuaded to put aside our efforts" if he were planning to veto the bill, Corcoran added. Evans and Corcoran, and 60 other congressmen who signed a letter in support of the measure, had voted to sustain the president's veto.

The Republican bill is a redrafting of an earlier effort that also had bipartisan support, although it was opposed by the Democratic leadership. House Majority Leader Jim Wright (D-Tex.), a strong advocate of synthetic-fuels development, led opposition to this proposal the first time around and is expected to be a strong opponent again.

The Republican plan would give the funds to state and local housing agencies, which would be directed to use them to reduce rates further on below-rate mortgage money they provide through existing tax-exempt financing programs. The details of the interest buy-downs would be left to the local governments.

The National Association of Home Builders, which strongly supported the previous housing subsidy bill, and the National Association of Realtors, which did not, indicated they would have to assess the real potential for passage of the plan before deciding whether to support it.