Great find! However, there might be a bit of rain on your parade ...
When finished the bike is going to vibrate like there's no tomorrow because there is no bracket to secure the top engine mount. You really will run some risks of cracking, brackets failing, etc.
The flat track frames made by C&J to the Yam factory race shop specs do have a top mount and were the primary frame choice of most XS racers.
I know it is blasphemy to say this, but it would be better to add provisions for a top mount.
Perhaps some sort of cleverly designed bolt on affair might work if you want to avoid welding at all costs?
Just sayin'