The gearbox is the most expensive part of that mower to repair or replace. If it is a sliding-gear gearbox, and the mower moves when you put it into gear, the clutch is not freeing, and you are sliding it into gear under load. That will destroy the gearbox quite quickly. I suggest you look under the floor, at the object circled here in yellow:
[Linked Image]

I'm guessing it is a bell-crank, and under the floor it attempts to move an idler pulley which tightens the belt from the engine pulley to the transmission pulley when you push the pedal. (Ride-on mowers are designed so they stop moving if you stop pushing the pedal. It's safer that way.) If that is the type of clutch it has, it sounds as if it is inoperative for some reason: probably the pulley is not releasing the belt tension when the clutch pedal is not pushed down, as it is supposed to. Please post pictures of the mechanism.

Vanguard engine production began in 1987. AFAIK, where Briggs engines have a Code, it follows the standard meaning. Briggs has sold various imported engines over the years, and some of them have not used the full Briggs model identification system (I think the Komatsu Zenoah 2 strokes sold under the Briggs name did not, for instance). The Vanguard engines were mostly imported, but since they have a Code in the standard format, I expect it has the standard meaning and that engine was built on 13 February 1992.

Here is the illustrated parts list for that Vanguard:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6osyGVJ1DajI.pdf

Here is the operator's manual:
http://bsintek.basco.com/BriggsDocumentDisplay/msquFO-K_ny7tH217FzoPu.pdf

In the early years the single cylinder Vanguards like yours were assembled by Briggs in the US, but after several years' production, assembly shifted to Mitsubishi in Japan. Then still later, in 2007, it moved back to the US. (The V-twin Vanguards were made in Japan by Daihatsu right from the start, and still are.) Your engine's Code does not match the Briggs plant which built the early Vanguard singles, so it appears to be a Mitsubishi-built unit. (The Briggs-built early Vanguards were put together at Menomonee Falls, Wisconsin, in Briggs' Plant 9.)

Last edited by grumpy; 03/06/13 05:34 AM. Reason: Add detail