Good choice those sp engines do not have that much power and parts are really hard to get. I had a sp250 last month and it was really bad. So I sold it.
It did start first kick tho ! But lacked in the power area. Wouldn't even yank the front tire in first gear.
Hence why a GL1500 or VT500 engine would be choice. I just bough my first 500 shadow a few weeks back and the thing has some serious power for it's size. Clutch is right there as soon as you let the handle go, no slippage, and I'm getting about 320mi per tank. It's a 6spd, 54hp @ 32 ft lb, Liquid cooled. Shaft drive so you can run a ATV rear end
For my 2 1500s, they're VERY torquey (0-75mph in 8.5sec) and are super smooth. Bit of BLEEP to work on but for reliablilty and performance, so worth the money. You can prolly find one for around $200-400 depending on year. 1994 is the MOST desirable and best pick out of all of the GL1500 I/A/SE line. They're 105hp, 111 ft lb, 5spd, liquid cooled. Need the upper frame section to mount it to though. Shaft drive also.
3rd best option would be a '79-82 Suzuki GS850. I4, 76hp, 52ft lb, ('81/82 Had CV carbs and EIS, '79 was point). It was alright for it's size, a bit laggy in high gear