That serial number I.D.s the mower as Honda HRM21PVA...dates from about 1991. It has a GXV110 Engine, (110cc) and is a standard OHV/pushrod design...some parts of the engine have long been discontinued from Honda, but many are still available.
Locked up engine could be an issue with the starter, or a deeper problem internally within the engine. Maybe begin by removing the starter and see if you can/can't get the engine to turn? Just a thought.
If you expect to really get into this mower mechanically, a shop manual is priceless. Honda sell 'em on eBay; free shipping and updates for 3 years, $35.00. Here's the link to one you need:
Honda HRM21 Shop Manual / eBay