I'd say to get a new (used) engine from someone, as the best combination and cheap and easy.
Another option, and the cheapest if you're mechanically inclined, would be to get a new engine case and replace that on this engine.
I would not weld it. I would *not* ride it for the summer and do it later.
How long have you had this bike? Has it been crashed, to your knowledge? You might want to look carefully for any other damage, and do a little research on the bike's history.
1993 Ducati M900 Monster "Patina" (265,000 miles, killed by minivan 30Oct17) -- 1995 Ducati M900 (wife's bike) -- 1979 Vespa P200 (daughter's scoot) -- 1964 Vespa GS160 (160cc 2-stroke) -- 1962 Maicoletta scooter (275cc 2-stroke) -- 1960 Heinkel Tourist 103A1 scooter "Elroy" (175cc 4-stroke)