Small Engine repair question...12hp-1 cyl riding mower, can you help?

Yes you should pop the sparkplug and look inside the hole if possible for metal shavings. Look at the sparkplug and see if there are any metal or aluminum shavings. Grab the crankshaft and turn it. Before you tear down the entire motor mayby you could just pop the head off and take a look inside the cylinder,mayby a valve come loose and locked the motor up. If the motor is actually seized up and you manage to break it loose dont think its alright to run again because most likely your rings are fried and it will destroy you cylinder beyond repair.Most likely the oil changes were neglected and over time the oil collected moisture and slowly wore the motor down till it seized. If you did it yourself you could prob do it for $125. If you had a shop do it 275.00 and a new or good used 12 hp motor $350-400.12 hp briggs should be a dime a dozen "used" so try a junk yard. Seeings its 15 yrs old you may be in the market for a new one ,check out ebay for a new or used one because you could prob find one for a resonable price.All in all it really sucks and if you lived in the same state as me i would loan you mine for a while too keep up the lawn but mine is just the "cheap" hydrostatic drive and not the "true" hydrolic drive. So it prob wouldnt fit in your "standards" of life. But if you are willing to step down to my level and drive a Deer i just may be of some help. Any way good luck ,live well and prosper.