125 RWHP from a 1500cc carbed SOHC will be a tall order for a "streetable" motor. Not impossible, but certainly at the upper limit of what availiable pump gas will support. Research all you can find on head work first, this is where the power is, and spare no quarter in this area, then select the apropriate cam for your expected range of RPM. Steve C. in Austailia has alot of great up to date info on what it takes to make a SOHC head flow. Run the piston to head clearance down tight (.035"-.040"), and the most compression you feel you can get away with on pump gas. With the proper length extractor tubes and intake runners, all tuned for the cam and RPM range, AND...all the other usual tricks (lightened flywheel, adj. camwheel, balanced, ect.) it can be done. But......it'll most likley cost as much as a budget swap at the very least.
Now.....if your Idea of a "steetable" motor is a little more liberal than most, like mine, and your not afraid to run a little 110 octane race gas now and then......you can run the compression up to 12:1 or more. This will REALLY wake the SOHC up and, with a little more cam......look out. The real advantage of a motor like this is throttle response, which will be instant.
Personally, if you decide to stick with the 1500, I'd suggest a modest turbo motor with programmable FI and ingnition. You can get the same, or more HP and torque as a well prepped hi-compression carbed motor, may cost a little more, but, you can tune it to run on pump gas. And with proper turbo selection and attention to pipe sizes and routing....turbo-lag should be a non factor.
In fact....I've just talked my self into building a turbo motor for my '74........well........I've started collecting parts anyway.
