I have just bought a 1983 T25 with a 1.9 petrol wasserbox. The vehicle has been standing for 2+ years

I put some fuel in and tried jump starting and the motor does one or two agonizing turns but stops, drawing current but making little noise, at the third compression of the cylinders.
I am told the van was driven to where it had been standing and the starter motor was fine then. Assuming the starter motor wouldn't fail without use, some bearings or other components must have ceased.
I am trying to figure out if it is better in terms or ease, time and cost to rebuild the engine or swap it for a a newer, working engine.
Is there much point putting time and money into a 30 year old engine? I'm looking to tour in it so a more modern, more efficient engine is appealing, although also thinking of doing an LPG conversion also
What do people recommend doing of the two options?
Many thanks!
ps. looking to use an AGG engine if swapping