@GomerP-
This may go against the flow a little, but bear with me. Without building a whole new engine-
To make more power with what you have it will need to rev a little higher, since 90 hp at 4800- 5,000 rpm is about right for a 1915. As already mentioned, finish sorting out the jetting, install 1.25:1 rockers and if the heaterboxes really need to stay, get a proper 1 3/8" merged header (preferably sidewinder design), take off your heads and get them properly ported and (at the same time) figure out your compression ratio and get them fly cut if need be. This would be a good winter project- start as soon as your driving season ends, taking the heads off and getting them to a head specialist. Line someone up well before hand so you're not just dropping them off and waiting 'in the queue' for weeks and weeks (or months).
The advantage- you're using what you have and everything you're doing will be well matched to the camshaft that's in the engine (we don't need to know exactly what it is right now), and as Ed said, you'll be hitting somewhere around 110 crankshaft hp (that's 1500cc 4cam power) with about 500 more rpm than now and none of the lack of bottom end (and drivability issues) that the 4 cam engines typically had. Carrera engines rev to 7,000 rpm with power and (especially the smaller displacements) really had nothing until the cam came on at 4500-5,000 rpm. You could just slap some Panchitos on it but they are kind of wasted here as they won't provide anything near their potential (160 hp or so) and airspeed won't be quite as nice at off idle/lower rpm's as stock ported heads, which are perfectly capable of 120-125 hp. You'll also get ever so slightly better mileage with the smaller valve heads- not that it's all that important, but it will tell you it's more efficient and the better combo.
To get a whole pile more power, as the guys have already said, you'll have to increase displacement by stroking it. A 21-2200 cc beast with Panchitos, 1 1/2" exhaust and a cam/rocker combo that revs to 6,000 rpm and gives close to 1/2" valve will make 145 or 150 hp and be an absolute blast to drive! (think big, evil laugh here!) Stan's advice of starting new and either selling the 1915 or putting it under the bench and Danny's words of wisdom are all spot on. It won't be cheap, though, and as Ed said, we do love spending your money...
Hope this helps. Al