SEATTLE (AP) _ A Vietnam veteran ended a 51-day fast Friday, saying he hoped he had ''lit a little candle to build a big bonfire'' after President Reagan agreed to meet with him about American soldiers who may be missing in Southeast Asia.

Gino Casanova had lost 45 pounds, but left his bamboo hut under his own power after being persuaded to end his fast 10 days early.

Reagan called the 34-year-old Tacoma man Thursday, agreeing to meet with him about the possibility of American prisoners still held in Vietnam.

''I feel real good,'' Casanova said Friday, but admitted his stomach troubled him and he was feeling dizzy.

He said he had done more than he had set out to do.

''It's amazing what missing a few meals can do,'' he said, making a joke about a ''bamboo cage diet.'' He drank only water with lime juice during his fast.

The 8-foot-square bamboo hut set up in a soggy cow pasture about 20 miles south of Seattle was enclosed by barbed wire to symbolize the cages used to hold American prisoners in Southeast Asia. Casanova said he believed as many as 300 American soldiers and fliers remain imprisoned.

Dr. Warren Appleton, Casanova's physician, said he was concerned about the man's health.

''He has an ulcer, he's real weak and he's lost about 45 or 46 pounds,'' he said. ''I'm concerned he will ... have a heart attack. His eyes are deteriorating for lack of vitamins. His liver's almost completely shut down.''

Dozens of reporters, photographers and well-wishers crowded around Casanova as he mustered the strength to speak.

His 8-year-old daughter, Shannon, stood with him, and his parents and sister watched from the side.

The crowd cheered as he was taken by ambulance to the Veterans Hospital in Seattle, where he slowly will be introduced to a solid diet. Casanova had hoped for a visit from Reagan when the president was in Seattle last Monday, but was satisfied when Reagan called from New York and agreed to meet with him and a delegation, which will include Rep. William M. Hendon, R.-N.C., within the next two months.

Hendon has introduced legislation calling for a congressional commission to investigate whether American soldiers are being held captive in Vietnam.

Casanova had planned to fast 61 days, with each day representing a Washington state resident still missing in Southeast Asia.

Casanova spent much of his time during the fast praying, talking to people who visited him each day and reading hundreds of letters he received from across the nation.

Wearing a flight suit and a knit cap, Casanova braved freezing temperatures and endured two severe winter storms with record snowfall of more than 17 inches in November in the area.