Hi Scott,
If you get a ID Plate marked "HK25" you can't go wrong as they didn't use the H4-4A with the primer bulb until the 1970's and as such the HK25's are all pretty much the same.
The only thing that differed was the smaller tank or the larger tank. I always liked the look of the larger tank but that was fitted mainly to the HK30 but also to the 25 in limited amounts. This would've been done due to the price Scott Bonnar paid for the engines at the time. Engine manufacturers generally offered discounts on over supply stock they had on hand to clear and thus if HK25's with larger tanks were cheaper one week then that's what they would buy a bulk purchase of.
there are a few of those available in Adelaide but as it's gotta go to Sydney this will raise the price dramatically.
You should be able find used units in NSW without too much drama I feel. The one main thing to take into account is the carby butterfly shaft and how much wear there is on it as thats the component that becomes very lose and thus induces hunting into the engine revs.
Cheers,
BB.
