If you are after a 4 cylinder in line then the BMW k 100-1200 rs motors would be the go. The big ass flywheel and car type clutch weighs a lot so bin that for reasonable weight. The have very big torque down low and are not a high revving motor for their power 100-130hp. The 1200 is best. All are bosch injected and very, very bloody hard to kill. They are nick named the brick, they are horizontal ie cylinders lay on side. Very compact.
A 100,oookm on one is considered run in, many have 300,000 plus and still strong as a "brick Shitehouse".
Yes they are watercooled. The 1200 motor can be had for $1000-2000 with everything needed bar a redrive.
The lario is a sweet little motor but it would be quite expensive to get one, quite underpowered even the brand new version at 750cc is considered gutless. Older models command stupid money due to cafe racer crowd and L plate legal.