In addition to the issues noted above, I would expect the battery configuration was chosen to reflect the allowed power. If power was raised (if it even could be) to the degree it would make a practical difference, battery life would be likely to suffer dramatically.
Well, avoiding questions of application and legality...
I suspect you are going to have a real problem finding something meeting the description you provided. However, given the difference in frequency, why not just use two antennas? It will still be hard to find much below 1 or 2gHz that would meet your needs for directionality. But it would be easier than finding a dual-band directional antenna.
However, I think you'll quickly run into the issue of diminishing returns. If you can actually find 2 antennas meeting your needs and also providing any useful gain, you will probably be in the $200+ range, per antenna and the difference in gain from a high-quality vertical antenna would probably be around 3db.
My guess is you'd be better off by spending your money improving your feed line and connectors and removing any adapters and then buying a high quality dual-band vertical which should get you gain of 8+dm VHF and 11+ UHF. (Eg. http://www.theantennafarm.com/catalog/comet-gp-9nc-1087.html )
You might play with placement of a reflector behind a dual-band vertical omni. But, that would really be hit or miss and you'd have to have the proper test equipment, time and interest.
berkinet's post in Motorola MR355R blister pack radios was marked as the answer