Top of the day. Thanks for adding me. I just picked up a 1979 TS 250. It needs some TLC (piston skirt is broken), piston and rings very sloppy fit in the cylinder; but the crank turns so it can't be that bad. I remember these bikes as a kid, and I hope to get it back to fully functional within a few weeks. I hope I don't hurt myself in doing so!
Check for up/down play on big-end and small-end. Best way to check the crank. Oh and rust + cranks don't mix!
These motors are very reliable and it is strange to have one catastrophically fail.
You are obviously up for new piston + rings, gudgeon pin + clips, small end bearing and cylinder base and head gasket + new sparkplug and cap (NGK of course).
And yea, sorry to be the harbinger of bad news, but I'd be splitting crank case and fitting new crank bearings and seals. May be new other bearings (o/p shaft probably) and bits.
New gasket set and parts mentioned with a little skill and you would pretty much have a fresh new RELIABLE engine. Just be sure crank and clutch nuts are tight!
Oh and take the barrel to a MOTORCYCLE engineer and have the bore checked. Don't skip this step. With new piston (possibly and oversize one), you WILL need a rebore anyhow. You may be lucky with only a hone to bed the rings in. But I doubt it. Costs about $100 NZD for this (rebore) and things will be fresh! You will NEED to give the engineer the new piston so He can fit it to the new bore...
With that over and done with, I hope the tank, forks, ex. chamber and seat pan are not rusty as buggery!
Oh and feel free to share a photo. You may edit your first post and include one. Please read his first...
ps. Being a '79, consider fitting some dual-stage Boyesens! They make a big difference! You will need to go 1, 2 or 3 sizes LEANER on the main jet though. I fitted them to my 185ER and 50X. Highly recommended. Cost about $20 USD...
pps. Ensure brakes are good and that tyres are good (for safety). -Don't ask!