I dont know, what fork/wheel/tire U are going to use, but if M1R, then U have to be carefull with distance between front tire and front sylinder head and radiator. Problems can come, when M1R fork goes very deep down in hard braking.
ST2 engine is just as wide, as 907IE engine. But, if U make "naked bike", then no need to be worried about wide of engine.
If U use 750 frame and add radiator, surely then need to make some welding job to make "connectors" for radiator to Paso frame. I think, ST radiator needs different mountings, than 907 radiator...

I give advise: If possible, go direct to curved ST radiator! :thumbup:
Let me know, how U "go on" with Your bike, cause i have some quite "close plannings". I look opinions to fit "some injected engine" to my 750 Paso and i have one spare 907IE engine(i have some other spare engines also waiting right time/frame...

) for that already, but ST2 engine is also very good opinion....