Datsun 1000 wagon

Just recently acquired a 1968 datsun 1000 wagon. I think it is a b10 or b20. The original motor is still in the car but is very tired. Attempted to drive it home last week(about 70 miles) but could only manage about 50mph which was way too slow for the motorway so I ended up trailering it home. My question was, is there a larger capacity engine with a 5 speed box that is a fairly straight forward conversion for this car. Have done several engine conversions but am not familiar with this model.

Depends if you are looking for something of similar age to the car.

A fairly straight swap might be a KA24 motor that was used in many different Nissan models through the 80s and 90s. It provides enough punch for just about any daily driver, came with a 5 spd in many models and it's pretty simple to wire. Might want to check the motor mounts on a few different engines to see what is closest to the car. Probably going to have to use the transmission from whatever engine you go with and get a custom drive shaft for it, which shouldn't be too expensive.