If the gun was originally made in 22 Mag. then you could probably fit a new cylinder safely. If not, I say forget it. H & R would likely have offered it in 22 mag if it was feasible because it would have increased sales. Top-break pistols are not the strongest and absent a factory version, I would not try it. It is simply not worth the risk. If you want to experiment, make a new cylinder in .17 HM2 and reline the barrel to .17 caliber. The cartridge is basically a .22 LR case necked down to .17 caliber and loaded with an ballistically efficient, jacketed expanding bullet. It makes a 150-200 yd varmint cartridge in a rifle plus accuracy is greatly improved over the standard 22 RF. Of course in a pistol, performance is reduced ,but it is still substantially better than the standard 22 RF.
Randy